It's time to take alternatives to dark matter seriously | Hacker News Reader

It's time to take alternatives to dark matter seriously

aeon.co

These articles always miss the mark because they get the history backwards. Modified gravity theories were taken much more seriously in the 1970s and earlier, where the evidence for dark matter was shakier. Cosmological and astrophysical observations from the 1980s to 2000s have vastly strengthened the case for dark matter, which is why it's the leading hypothesis now.

Popular articles won't tell you this, because without exception they laser focus on galaxy rotation curves, a piece of evidence that's nearly a century old and by far the weakest one. But by neglecting to mention the actual evidence we base our conclusions on, they (purposely or not) make us look like fools.

Furthermore, dark matter hasn't been ruled out. One particular candidate of what it could be (a WIMP) has been studied and mostly ruled out over the past 20 years, but by the nature of the business there are many possible candidates. And of course we've shifted attention towards those other options, which is exactly how science is supposed to work!

You’ve also neglected to mention the evidence that the articles are leaving out ;0

For example, galaxy collisions. Dark matter by its nature doesn't collide, so it behaves differently than the galaxies it's in when there are physical collisions, and that's visible. Basically, the galaxies stop; the dark matter keeps going.

We can't directly see it, but we can see the gravitational lensing it causes. Dark matter isn't as invisible as believed.

Then there are cases of galaxies which don't contain any dark matter, and therefore act the way we were expecting them to from simple gravitational mechanics on visible matter. That rules out any solution involving changes to the laws of gravity, since such a change would be universal. The fact that galaxies can exist which contain none means it must be a substance of some kind which can exist more in one spot than another.

Why does the dark matter stop? Don't colliding galaxies stop due to gravity, which dark matter is equally affected by?

Galaxies are drawn toward each other due to gravity, but when they meet ordinary matters "stop" because lumps of gas plow against each other, causing them to lose energy and stop.

Dark matter doesn't interact with anything else (or even itself) so they pass right through each other.

So does that mean there are some galaxies without dark matter - because of a previous collision? And then some dark matter only galaxies, because they left their galaxy behind? I suppose this is unknown - but does the dark matter reappear in the galaxy later somehow, or it's left behind forever?

Edit: well there are galaxies without dark matter (maybe) [1] and there are dark matter galaxies [2]

[1] https://www.space.com/19-galaxies-missing-dark-matter.html

[2] https://www.space.com/33850-weird-galaxy-is-mostly-dark-matt...

reappearance is possible in collision sims, but the timescale is too long to be easily observable.

Same reason cars stop when they hit a brick wall.

Most of the mass of galaxies isn't in their stars (which, for the most part, keep moving with the dark matter) it's in gas clouds and dust formations. When two galaxies collide, the gas and dust effectively halts.

What we see when galaxies collide is that most of the mass of the galaxies move straight through each other without appreciably decelerating. And most of the matter, the gas clouds and star forming regions, halt in the middle.

MOND was taken seriously as a theory from its inception in the '70s through the end of the 20th century, but because of the bullet cluster, it is effectively dead. MOND is falsified by observational evidence.

For this article to discuss MOND so thoroughly, but not mention the bullet cluster once... It doesn't pass the smell test. This is pseudo-scientific trash.

There's other evidence against MOND and various alternative-gravity models (there are several of them, it's a large umbrella) but most of them do a very poor job at explaining the edge cases, such as the bullet cluster and ultra diffuse galaxies. Some ultra diffuse galaxies have a conspicuous absence of gravitational lensing and stars which orbit extraordinarily slowly; ie, they have very little dark matter. If the explanation for dark matter was that we had the laws of physics wrong, the newly revised gravitational theories should apply equally well to these galaxies- which they don't.

While the article is perhaps not written as well it could have been, alternative gravity is probably indeed bust, the point that other theories need to be funded and studied is valid. it is not a either/or scenario.

The argument the article makes is that just because emergent gravity or many alternative candidates potentially cannot explain all of evidence that dark matter can , does not mean it should be thrown out completely and rejected, it could be still funded and studied to see if it can explain some of it.

The real issue is given the way modern academic funding works we have been tending more and more towards narrower research with main line theories and lesser riskier proposals perhaps being incentivised. This is not just limited not dark matter , this phenomenon is evident in string theory research and many other fields as well.

How about we fund somebody like a George Church to just a have a lab and pursue whatever he feels like in cosmology instead of such a narrow "school of thought" picked by a donor?

Maybe, if the donor feels like it... I mean, even if dark matter is the theory most likely to be correct, it's not as if further research along that path will yield any kind of monetary "return on investment" in our lifetime, so researching alternative theories is no more or less a waste of money :)

No, the conclusions are not valid. This person doesn't know the most important experimental results in the field, doesn't understand why modified gravity is both theoretically and experimentally unsound, doesn't understand that dark matter research is already not a mono culture, doesn't understand that detailed investigations of gravity potentially diverging from the inverse square law already exist and that new studies are planned.

This is the equivalent of someone reading some nasty things about C and claiming we need to investigate python for kernel development.

As an aside, you've put the cart before the horse here. When you have multiple competing theories, you don't study them separately. You design experiments which will (hopefully) measure different results depending on which theory is true. This is exactly what is happening in cosmology. The experiments have ruled out MOND and their like. New results will rule out more dark matter candidates. Surprising results will lead to new theories.

the problem is, so much is based on circular assumptions of theories, most of which is bullocks.

cosmology was convinced that the universe is homogeneous and isotropic (the universe is about the same everywhere). they we learn that the rotation of galaxies is not random...

the universe is flat, and then its not.

the expansion of the universe is the same everywhere... and then it might not be...

we had one way of measuring the age of the universe with method a, and another with method b, and they use to have overlap. and now they dont... https://youtu.be/cnn_5YMpo3Q?t=876

Honestly, if we agreed that we know 10% of what the heck is going on out there, that is probably being way too optimistic. so to blindly parroting "that's pseudoscience' to cast doubt on something that hasn't been observed in 60 years despite supposed to be representing the majority of the universe... that feels like religion, not science

> the problem is, so much is based on circular assumptions of theories

What specific circular assumptions are you suggesting?

OP isn't calling the concept of MOND or alternate gravity theories pseudo-scientific, they are calling the article pseudo-scientific.

Such opinions show a lack of understanding for how science works. Yes, scientists come up with hypotheses all the time, with the only requirement being for them to be falsifiable.

And the theories themselves can't be proven as true, since this isn't math but empirical science, but we can get an idea about how well they predict natural phenomena and we can disprove them by empirical evidence, observations that show these theories don't predict the universe well.

Scientific theories are a dime a dozen and in the presence of empirical evidence that falsifies them, we should consider such theories as falsified and move on.

When people continue believing in theories that are non-falsifiable or that have been falsified, that's pseudo-science.

Putting the OPs remarks to one side, that's a rather simplistic view of science.

> And the theories themselves can't be proven as true, since this isn't math but empirical science

This distinction is meaningful and extends to logic, metaphysics, etc.

> When people continue believing in theories that are non-falsifiable or that have been falsified, that's pseudo-science.

Popper distinguished between rationally justified claims and "scientific", i.e., empirically testable claims. He only claimed that falsifiability was a feature of empirically testable claims. That he should do so should be obvious because the claim that all claims must be empirically falsifiable is not itself empirically falsifiable and large swathes of human knowledge lay beyond the empirically testable, hence the restriction. That they are not empirically falsifiable does not make them pseudoscience. Indeed, the viability of empirical science hinges utterly on unfalsifiable assumptions.

> Scientific theories are a dime a dozen and in the presence of empirical evidence that falsifies them, we should consider such theories as falsified and move on.

Not so fast. Even within the domain of the empirical sciences, falsification is not simply a matter of performing a tidy critical experiment. As Duhem (and Quine) observed, the theory under scrutiny hinges on many other hypotheses, theories and assumptions. Even something as simple as observing something under a microscope hinges on the soundness of optics as well as many other assumptions that are not falsifiable.

There is most certainly an economy, a parsimony, an order to how science and common sense proceed when met with conflicting evidence. We tend to doubt the the outer layer of the epistemic onion where there is less certainty before asking questions about our assumptions.

And of course, science occurs in a social context such that it is subject to practical and cultural forces that influence what is given priority. In any case, the point is that falsification is not simply a matter of demonstrating that an expected outcome does not occur.

> but we can get an idea about how well they predict natural phenomena and we can disprove them by empirical evidence, observations that show these theories don't predict the universe well.

Predictability is only part of the story. If we reduce science to predictability, then we're back to a purely instrumental view of science. But as Hillary Putnam notes, science aspires to describe reality as it is. It does not merely want to produce a model that "works" which is to say one that allows physical phenomena to be predicated. In any case, that predictability should be a feature of empirically testable claims is not falsifiable, and predictability itself presumes that the universe is a certain way that affords predictability which is, again, not falsifiable.

Quoting from the Article: 'it will force us to decide what kinds of evidence we need to believe in" ?

But -correct me if wrong - today there is one known kind of fusion that -cause-(crossed out) leaves us with "Anti-Materie" as reaction (remembering the 1st art law, saying 'Energy is a Sum') ? (-;

Off-topic, would you have any recommendations for introductory reading on these topics? I’d be very interested to learn more

Hi, you sound like you know what you're talking about.

Can you elaborate with more details (my "reading comprehension" is such that I enjoy/comprehend academic papers more than watered down (and sometimes wrong) "pop-science" simplifications) about the counter-evidence against MOND?

You've given me some great topics to google search for, and it's much appreciated! (somehow I missed hearing about the bullet cluster. reading about it now... including the rebuttal: https://web.archive.org/web/20160721044735/http://www.astro....)

Can you perhaps drop a few more links to things I can dig into that provide counter evidence for MOND? MOND has been sort of a ... romantic crush ... of mine since I first learned of it 20 years ago. :)

I think the fact, that most of the galaxy mass is in dust and gas clouds is the most important bit here I didn't know.

> Why does the dark matter stop?

The dark matter doesn't stop; it keeps going.

> Don't colliding galaxies stop due to gravity

Not really. As others have noted, they stop due to non-gravitational interactions between the ordinary matter (mainly gas and dust clouds).

"Then there are cases of galaxies which don't contain any dark matter..."

You mean zero at ALL, or with little measurable effects?

We don’t know, more likely the latter.

We can see that some galaxies rotate as if they had no dark matter, but we don’t know why their dark matter isn’t there.The prevalent, but still speculative, hypothesis is that it was “stolen” by a bigger galaxy in a close pass-by.

So does dark matter interact with itself outside of gravity? If not, how does the visible matter separate from dark matter.

If you imagine that dark matter and visible matter both start out basically smoothly distributed in a big cloud, they'll both tend to collapse inwards. The matter forms a rotating disk because the particles of matter interact through non-gravitational means; so ones that get close can exchange (and cancel!) momentum. But if dark matter only interacts gravitationally, it won't do this; the particles will just fall through the center of the future galaxy, 'passing through' each other (and the matter). Gravitational drag presumably adds some rotation to the dark matter over time; I don't know. But that's how dark matter and regular matter could separate.

>So does dark matter interact with itself outside of gravity? There are different ideas about what dark matter could be. Some have it interacting through other forces (WIMPs interacted through the weak force, for example), some are purely gravitational. There's always talk of 'dark matter candidates', theoretical or otherwise particles that could plausibly be dark matter.

- gravitational lensing measurements

- velocities in galaxy clusters

- getting the fine details of the complicated, wiggly cosmic microwave background spectrum right to extreme precision

- getting the fine details of cosmological structure formation (as measured from matter density fluctuations on very large scales) right

- galaxy collisions, like the Bullet cluster (hard to explain with modified gravity)

- the fact that some galaxies have much less dark matter than others (very hard to explain with modified gravity)

For the last point. Where do we get our confidence in the (light) mass estimation of a galaxy? Couldn't we be off by a factor of 10?

Mostly the confidence comes from half a century of trying to find such "extra" visible mass! There's a long history of people looking for extra dust or gas or compact objects using a tremendous variety of clever techniques, and yet the deficit has remained. At this point, if you believe that there indeed is extra mass around, it kind of has to be dark matter by definition because it's been so damn hard to see.

The question is how the f do we know the mass of a galaxy in the first place? We can't have the discrepancy until we have two distinct methods of measuring the mass of a galaxy.

The best evidence for dark matter is arguably the CMB. The strengths of the individual peaks are highly suggestive of dark matter, in particular the third peak. Furthermore, most MOND simulations on cosmological scales still require dark matter (if they evolve the Universe following the Friedmann equation), for example an 11eV sterile neutrino, because there has yet to be an underlying theory that accurately describes the evolution that can reproduce the mass function and galaxy clustering with MOND. This isn't to say that dark matter doesn't have its problem, one of which is the fact that we don't know what it is; however, there still remain numerous candidates that have yet to be sufficiently explored experimentally.

Is there a scenario where both could be right in some way, so part due to DM and part due to modified gravity?

Modified gravity is still being actively explored and I would argue taken very seriously but not in the context of dark matter, rather in the context of dark energy. So in a sense, yes, modified and dark matter combined are both possible in the same theory. However, at the moment, there is no real reason to suggest both dark matter and modified gravity are responsible for things like galaxy rotation curves because one of these works just as well without the other. In fact, many modified gravity theories apply "screening mechanisms" so they don't impact galaxy scales

Sabine Hossenfelder has a great series of videos about both dark matter and modified gravity theories (https://www.youtube.com/watch?v=FN2d2cmi_Gk, https://www.youtube.com/watch?v=2VNcDoLNJk8, and https://www.youtube.com/watch?v=468cyBZ_cq4). I think she does a very thorough (and relatively easy to understand) job of explaining what both of the types of theories are, the motivations for developing them, and their strengths and weaknesses. The first video is about dark matter, the second is about modified gravity (mostly focusing on MOND), and in the third she talks about how she thinks it makes sense to accept both. Even if you don't agree with her position, her videos are still very informative for anybody who wants to know more.

Question: as a layman, knowing nothing about the quantitative nature of the evidence, neutrinos seem like the perfect candidates for dark matter, since we've already detected them and we know interact so poorly with anything. Why are they not the obvious candidate for dark matter? Is there any evidence against them?

they are obvious. The evidence against it is that there are not that many of them (or we would have detected more), and while we aren't sure how much they all weigh we're pretty sure it's orders of magnitude less than what would be required for dark matter.

I believe I heard on a podcast that there's a theory that dark matter might actually be multiple things, and that neutrinos could just account for a small percentage of that. Is that correct?

That's correct. In the standard ΛCDM cosmology, neutrinos would provide about 0.5% of the required matter. See the Particle Data Group's dark matter review: http://pdg.lbl.gov/2020/reviews/rpp2020-rev-dark-matter.pdf

Neutrinos move too fast. Dark matter ought to move at nonrelativistic speeds to generate the observed effects. Another way to put this is that neutrinos are "hot" dark matter and we expect "cold" dark matter.

That's a perfectly good question, and that's exactly why for a time in the 90s they were one of the leading candidates for dark matter. Unfortunately it fell apart when we got a better understanding of structure formation, which determines the large-scale distribution of matter in the universe today. If the 3 neutrinos have masses around where we think, then they'd be too light and move too fast in the early universe, smoothing the distribution out and thus suppressing small-scale clumping, contrary to observations. On the other hand, if the 3 neutrinos were a lot heavier (possible we can only detect their mass differences, not the absolute masses) they would screw up the CMB. The qualitative case was great, but quantitatively it just didn't work out.

I'm not an astrophysicist and well outside my depth here, but couldn't observed clumping with low-mass neutrinos suppressing clumping be explained by an older universe as well? Put another way: if clumping is suppressed, that doesn't mean it doesn't happen, it just means it happens slower than initially thought, no?

Also a good idea, but the details don't work out. Specifically, early on the Fourier components of the matter density distribution grow independently. Light neutrino DM would suppress the small scale clumping too much. Making the whole process last longer would fix this, but it would also enhance large scale clumping, contrary to observation.

It might be possible to make it all work out by adding a third effect (and maybe somebody has already done that!), but there's also a danger of constructing a Rube Goldberg machine -- if there are too many gadgets needed to get the structure right it starts looking like epicycles. The great thing about traditional dark matter is that it doesn't require any cosmological complications, it just works.

So lemme check my understanding here: are you saying that clumping has an observed size distribution, and if dark matter were just neutrinos, there isn't a timescale which would produce the clumping size distribution we see: if the timescale were short then we would see less small clumping than we see, if the timescale were longer then we would see more large clumping than we see?

Neat.

Neutrinos are the obvious candidate. To make long story short, the evidence against is that numbers don't work out. I know it is unsatisfying to those who can't derive numbers themselves, but that's what it is.

We know how many neutrinos should be there, and it's not enough to provide a dark matter candidate. Additionally, neutrinos would be hot dark matter, that is they would move fast, and therefore not cluster nicely. That would lead to galaxies that are more spread out than we observe.

(That is, as long as we assume standard neutrinos, there is actually a theoretical mechanism to get heavy right handed neutrinos, while leaving the left handed ones we observe light, and that could be kinda sort of a dark matter candidate. The most obvious way to generate them is as far as I know ruled out, but I am sure people work on more complex models.)

We need better Modified Gravity theories, because 99% of the time people say "waa MOND was disproven already", yeah, actually "The MOND theory" (the modified gravity that everyone calls MOND) doesn't work out.

Doesn't mean any other formulation wouldn't work out.

I used to work on dark matter experiments. (This was a few years ago now, and wasn't for very long -- I was helping out colleagues -- but I don't think the situation has changed much.)

I don't think anyone worth listening to considers dark matter to be a Universal Truth or anything. It's a crappy theory that needs better experimental evidence on the micro scale. It's just that all attempts to find that evidence have turned up nothing. (Except DAMA/LIBRA, the bane of the previously mentioned colleague's days and motivating factor for the experiment in question... but nobody actually believes them, so let's value their reports at "nothing".)

The biggest problem is that all alternatives to date are worse. They all have problems, problems of equal or greater severity than dark matter. That, more than any other reason, is the reason dark matter is still the leading theory.

I do strongly believe we should work on the advancement of non-dark matter theories. That's how you lap the current leader, after all. And neglecting alternative theories was what got us into a couple decades of string theory stagnation, a mistake we'd be wise not to repeat. If a new theory or new evidence for an existing theory emerges, dark matter will lose mindshare amazingly quickly.

There is, of course, a "nightmare scenario". The WIMP carries a weak charge, by definition. It was dreamed up that way to make it detectable and have a presence in the universe. (And there may have been a theoretical preference; I wouldn't know, I've always been an experimentalist.) But there's no reason it must carry a charge, weak, strong, or electromagnetic. That would make it really quite invisible to experiments. This may be the universe we live in.

But doesn't it have to have some charge to interact with matter in some way so as to achieve the observed effects that show that gravity from normal matter is inadequate to explain how things move on a macro scale?

It only has to have mass for gravitational interaction, and other charged (color, weak, em) are (mostly) excluded by observations. If there were much beyond gravity going on, we would have seen it. So our hope of detection is the remaining "not much"...

> have to have some charge

it can also be a new interaction driven by a completely different fundamental force. it's extremely unlikely, but "have to" is quite a strong statement for something we only know exists because observations deviate from our expectations.

How do you even judge the likelihood of something like that?

same as everything else, the more changes it requires from the current model the less likely it's the solution

That's not how I judge likelihood at all. If you were to say that there is more mathematical wiggle room for other explanations, I would be more likely to agree, but simply having to change models has no impact on likelihood. That thinking would make Quantum Mechanics, Special Relativity, Natural Selection, or Genetics unlikely. Your argument could even be used to explain why it's more likely that the earth is flat because that's the intuitive model.

unlikely doesn't mean it's less good of a theory. it means the bar for verification/falsification is so much higher.

I just find this odd... we've seen Dark Matter fill in the gaps of standard astronomy (general rel) models of galactic evolution, but it's been predicted and seen in multiple places over the last 50 years and the measurements keep getting stronger (cosmic background spatial distribution, galactic rotation, bullet cluster). Each measurement rules out some model explanation, and we search for different particles (WIMPs, MACHOs, neutrinos) that fit the models that work.

We still have interesting testable explanations... but let's give up now, it's boring? MOND again?

Sorry, axions are interesting and may explain other things (antimatter ratio etc) and if that fails I think others will pop up, that don't require reexplaining astronomy.

https://phys.org/news/2020-06-case-axion-dark-gains-traction...

I think an argument can be made that we should have given up (figure of speech: rather, distributed most of resource to other experiments) on WIMP much sooner. These days we agree "WIMP didn't work out", but in retrospect, was it actually the most promising prospect for dark matter, even in 2010? Why was XENON a better experiment than, say, ADMX? I mean, if it were up to me I would fund both, but I think there was certain inertia and undeserved bias for WIMP.

The very nature of the study of physics has created a long history of untestable theories to fill in the gaps. Because candidate theories that can be tested either solve the problem or remove themselves from contention, so until the problem is solved, the untestable ones are the only ones that last any significant amount of time.

The theory of luminiferous ether had a large amount of circumstantial experimental evidence for quite a long time, but all it really was, was a placeholder waiting for an actual explanation. Theories like that will always exist on the frontiers of science. It’s not really unique to physics either. Miasma theory played a similar role in the history of biology for instance.

> But now it’s time to consider...

Almost invariably when an article title says "it is now time to..." there has been no actual threshold that has been crossed making "now" special at all.

The growing types of evidence for a "dark matter" effect on large scales, vs. the absence of any small scale evidence for it, has consistently begged for new particle, gravitational, and other types of theory to be explored.

It is a strange situation to have something appear so consistent with one of our pillars (general relativity), while completely absent from the other (quantum mechanics).

I am crossing my fingers that any successful understanding of dark matter on the small scale might shed light on how those two pillars could be combined into a single theory. But any explanation will be most interesting!

It's pretty easy to get a dark matter candidate out of quantum mechanical theories of elementary particles. The current leading candidate, axions, was proposed all the way back in 1977 for reasons completely unrelated to dark matter.

If you enjoy science fiction, one of the better recent SCPs was about dark matter:

http://www.scp-wiki.net/scp-4170

SCP is a world-building collaborative creating-writing project, focused on sci-fi cosmic horror. Each entry is standalone, though there is also some canon which contributors are encouraged to follow.

SCP-4170 was generally received as a positive entry, though I read it as being far more sinister.

Miriam: [Request to still be taken seriously]

That really was a good one. Thanks for linking it!

MOND was a healthy idea to pursue, not because it was the correct alternative (I really have my doubts that it was) but because it was AN alternative.

Maybe some postgrad might have had less of the mainstream science groupthink indoctrination because of it.

That being said, dark matter is a real phenomenon: it is a measurable quantity of how wrong we are. Dark matter isn't the problem, WIMP is.

We have a theory and observations. Observations don't match theory. So, at least one of them is incomplete.

Dark matter puts the emphasis on observations(that is: assume the overall theory is roughly correct): there are things out there that, if we would have observed them, would make the theory fit.

Alternatives for gravity focus on the theory (accepting observations as roughly complete): if we change the theory, it should fit the observations.

TL;DR we ought to have A=B, but there's a mismatch. The problem could be due to either A or B. Or both.

Both should be worked on - at least until one can be fully ruled out.

Doesn't the bullet cluster provide strong evidence for the existence of dark matter? Shouldn't all alternatives need to explain the phenomenon seen there?

Iirc it provides evidence against most modified gravity theories, but other alternatives could theoretically still be viable.

It provides strong evidence of matter that we cannot detect. Milgrom's reply is worth a read: https://web.archive.org/web/20160721044735/http://www.astro....

Thus... dark matter?

Isn't dark matter something like a shadow DOM.. It's there but also not there?

Edit! Wow really feeling the love on this comment

What I mean is dark matter is different from matter we just can't detect, it's matter that should be there but we can't seem to find it

It's pretty simple: Dark Matter can't be seen or felt. It probably just travels through regular matter without "colliding", muchlike neutrinos.

It only interacts with other matter through gravity.

Now, we can detect gravity, but only for fairly big and stationary objects. If DM is tiny particles travelling fast, we can only detect aggregate effects on galaxy level.

Yes just like we can't detect the shadow DOM from the DOM but it's really there you know? I feel like my thinking is too orthogonal any more.

There's a difference between matter that's there which we can't detect as opposed to matter which isn't there which we can oh geez

If you're making sense, I can't detect it :)

Maybe it is similar in that the shadow DOM predicts the state of the actual DOM. In this interpretation dark matter predicts the shape of our universe. It might just be how one should go about simulating a universe..!

No. Because dark matter is understood mostly to be less interacting matter. Don't think it's a ball of dark stuff, think something you couldn't even see

Whereas he's proposing regular (dark as in color) matter

> Whereas he's proposing regular (dark as in color) matter

Wouldn't that kind of matter be easily detectable because it would block EM radiation, including visible light?

Yes. "Dark" is misleading, it should be "sparsely-interacting" or something. Well, MACHOs would have been dark in the traditional sense as well. Generally, "we would have seen it" excludes a lot of dark matter candidates, thats what actually makes it "dark"

That's my understanding as well.

Yes, the bullet cluster effectively disproves most theories like MOND and similar. This article doesn't mention the bullet cluster. The article is dead on arrival as far as I'm concerned.

Additionally, some galaxies have dramatically more or less dark matter than other galaxies. There's not a very good path forward for MOND-like theories with regards to explaining these galaxies.

https://en.wikipedia.org/wiki/Ultra_diffuse_galaxy

> [Milgrom's alternative theory:] But far in the outlying areas of the Milky Way, stars would feel a smaller gravitational force than previously thought

Isn't it the opposite?

Dark matter is extra matter which implies extra gravitational force to hold the galaxies together. If you want an alternative to dark matter, shouldn't the alternative theory predict a bigger gravitational force than previously thought?

So without dark matter explaining the planets not flying out of the system, there is actually less gravitational force, yet they are obviously still not flying out. Why? Simply due to the shape of space.

As far as I can understand, you are correct. The problem is that predicted gravitational force at the edge of the galaxies is smaller than observed, so any theory should find a solution for that problem. MOND in my opinion always looked like trying to fit a solution to the observations which will lead to overfit.

I would enjoy an explanation as to why the distribution of normal and dark matter in galaxies is roughly the same. My ignorant intuition tells me that since normal matter is subject to many more and different forces than dark matter, one of them should be much more densely-orbiting the galactic core than the other. But they aren't.

Maybe dark matter is just a symptom of the universe’s connection to the multiverse. Externalities in action.

Gravitational effect from nearby parallel universes maybe.

I'm wondering always about the name "dark matter". It makes a good topic for all kinds of pseudo-science and scifi. The name implies something mystical and invites everyone to discuss a topic that no-one understands.

If the name was something else like "Rubin's matter" or "Unknown matter", maybe it wouldn't invite so much unscientific discussion and speculation.

I don't feel that everyone has an opinion about higgs boson, but you can start a discussion about dark matter with anyone. I'm not any kind of expert about the topic either, but could certainly start speculating about it with other laymen.

It’s literally dark.

> But far in the outlying areas of the Milky Way, stars would feel a smaller gravitational force than previously thought from the bulk of matter in the galaxy;

Shouldn’t it be: "a stronger gravitational force than previously thought"?

A simple back-of-the-envelope estimate shows that it takes an -extremely- small amount of very low-density 'invisible' matter - assuming that it's spread out over the huge 3D 'spherical' volume of a galaxy - to make up the missing mass. On the order of a few -atoms- per cc.

Given all of the unknowns about interstellar/ intergalactic matter, including the density of EMF, it's not too hard to understand why the account sheet won't balance. It's a hard and wonderful problem.

What if dark matter is really just regular matter that is not visible through our instruments?

Like large swaths of ice or dust in the voids, similar to our kuiper belt that is difficult to analyze.

There are some clever ways of detecting such things. The density of gas and dust can be determined by measuring the effect on starlight passing through it. Statistically, larger objects should occasionally pass in front of background stars making them "blink off" briefly or in the case of black holes, lens the light from the background star, making it briefly brighter. Surveys have been done looking for such things. We can put upper limits on the amount of "traditional" matter, and there simply isn't enough of it out there to explain our observations.

https://en.wikipedia.org/wiki/Baryonic_dark_matter

What, exactly, is the prediction of dark matter, compared to MOND (or related theories)?

As a layman it seems like:

1. Dark matter is not universal (i.e. it is a "random" property of space) 2. Dark matter interacts with itself (and other matter) via gravity.

Is that all there is? I mean in such a case it seems to be no wonder that it fits the observations better, as DM theory is by far less constrained then MOND.

While the stationary background universe model proposed by Peter Ostermann doesn't rule out „dark matter”, it at least doesn't require „dark energy“ in order to match with observational data.

http://peter-ostermann.org/assets/sum_os14.pdf

MOND is appealing from a purely philosophical point of view: we have a separate (“modified”) theory for when thing are very small, another one for when things move very fast, so, the need for another modification for when things are very large should not be seen as entirely unexpected, should it?

We have none of these things. We have theories which say interesting things about small and fast things. But the theories don't change when things are small or fast.

MOND is excluded experimentally (violates observation of galaxies with and without dark matter). It is unsound theoretically.

Does anyone know if there are any effects from inflation [1] at galactic boundaries?

[1] https://en.m.wikipedia.org/wiki/Expansion_of_the_universe

I've often wondered if there is an equivalent of the magnetic field for gravity. The magnetic field seems to be a consequence of resolving charge interactions for relativistic observers, and the same could be true for gravity.

Can anyone Please point to the model used for the "expected" galactic rotation curve? This whole dark matter thing started with observation not matching prediction. I'd like to scrutinize the prediction.

newton's theory of gravitation was the model used for predicting expected galactic rotation curves

Plus mass distribution of bright matter inferred from brightness curves.

It is a simple enough integral to do, grab any beginner's astronomy book.

Hope this isn't a stupid question, but does dark matter have different states like solid, liquid, gas and plasma?

“Dark matter” is the name of an observable effect. We do not know if it’s matter, even.

Is it possible that the effects of dark energy are in reality a previous universe (like an outer sphere) that is pulling our universe matter?

You are reading too much Ian M. Banks. (Note: that’s fiction, not science.)

Never read Ian M. Banks. This is just a question. If not possible why?

Yes! I've been saying for years that dark matter is just a mathematical convenience and that we don't have any real evidence for its existence. Our model w repsect to it needs rethinking

Maybe physics needs a more holistic light theory based on stillness and the two-way motion of each phenomena in the cosmos: https://wikischool.org/divided_light

Haha what on earth is this. Every article starts with a bunch of quotes from some crank with no scientific training. Reminds me of https://vixra.org/.

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