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Comment by superqd | Hacker News Reader

I very much wish no one had used the terms "dark matter" and "dark energy", as those imply far more knowledge than we have. It implies to those unfamiliar that there is in fact some form of energy or matter that has been "sort of" identified, but whose properties are unknown.

That isn't the case at all. We have equations we use to predict observations, and in certain cases, those observations are not quite matching what we predicted. There are a lot of reasons why this can happen, and using the equations poorly is but one of them.

I really wish they'd just said we've found an inconsistency, between theory and observation, which is the actual case. It would, I think, better inform the public about the actual process of science, rather than making it sound like a new discovery has been made, which has not happened, clearly. Or perhaps the real discovery was the inconsistency, and that should be the framing of the problem. Describing it using sort of known words like 'energy' and 'matter' makes the general public think a new type of matter has been found, rather than what has actually been found which is a hole in our understanding.

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There's inconsistencies, and then there's real inconsistencies.

Recall some predictions that were later confirmed: the top quark (18 years later) or the Higgs boson (48 years!). These theories were a jigsaw puzzle with a single piece-shaped hole. You could call "inconsistency!" and break apart the whole puzzle, or look behind the couch for the missing piece. (The piece was found!)

We also have real inconsistencies, such as between QM and GR. Not a missing piece but a yawning gap: no single observation can fill in that hole.

Dark matter is a "missing piece" problem. The dark matter hypothesis fills in the cosmological missing mass problem, galactic rotation curves, Bullet-cluster-style gravitational observations, the expectation of non-EM interacting particles, and others. The gap between prediction and observation is the size and shape of a single jigsaw piece; let's just find it behind the couch.

Dark energy is a "real inconsistency" problem. Predictions and observations are orders of magnitude apart. Well hell.

Here's an image explaining the "bullet-cluster" observation. It's a compelling piece of evidence.

Two galaxy clusters have collided. The hypothesis is that normal matter will be slowed down by the collision, while dark matter, not interacting (as strongly?) with itself or normal matter, passes through in the collision. We can determine the locations of the gas by X-ray telescope, and the supposed location of the dark matter (assuming it exists) with gravitational lensing.

https://ircimg.net/slide_33.jpg

(I am not a physicist, just had this explained by a physicist friend of mine).

Indeed, this is the observation most usually cited in support of the matter / particle interpretation of dark matter. It's very non trivial to come up with a decent theoretical extension to GR that explains it. Which is why, I think, that a lot of people feel more confident proposing a new dark matter particle, which does in fact explain it quite nicely.

Perhaps we're wrong in assuming that all of these observations on very different scales (galaxy rotation curves over galaxy cluster collisions to large scale structure) are tied to the same unknown (dark matter), but if you want to explain them all in a simple way, a particle still seems like the way to go.

Remember that elementary particles have been predicted and found many times over the past years, whereas modified theories of gravity haven't seen the same kind of success. Of course this is only anecdotal, and may not mean anything, but I think it's part of the reason for most people's preference for a particle explanation.

The final bullet point in that image contains the answer to the question of dark matter.

The counterargument given by one of the proponents of MOND (a non-dark-matter counterargument) is that the bullet cluster can be explained by a combination of slight modification of GR, and additionally by the presence of a significant (but not crazy) amount of cold regular matter. Cold regular matter can't be easily detected because it isn't emitting any light, but it is just regular matter.

I'm not actually familiar with the specifics so I don't know whether this is actually a good explanation, but it's at least worth discussing.

This is a good explanation, but I'm not so sure the missing piece is so clear in the case of dark matter (and the blog post explained this nicely).

Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results.

Maybe gravitational attraction is not r^2 at some non-obvious scale that we failed to measure (meters? mm? Casimir effect on cosmic dust? Solar system sized gravitational anomalies?)

Why relativity makes Mercury's orbit precess is very not obvious (I mean, the two-body problem is already hard without relativity). The bending of light is a more obvious effect for example.

Right. A lot of "modified gravity" theories have been proposed in recent decades, investigated, and compared to observations; with work they can be made to match up. Dark-matter theories on the other hand do a somewhat better job of matching observations, which is why they have become the best-liked explanation for the galactic rotation curve puzzle.

Scientists didn't just jump to dark matter at the start, and no cosmologist will tell you that dark matter is definitely the explanation — it's simply the explanation that currently seems most likely.

> Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results.

How do you propose for a modified equation to explain the bullet cluster? That is, we have multiple good observations that show situations where the distribution of mass differs substantially from the distribution of visible matter, and good theoretical explanations why this should happen in the case of dark matter. This is not adequately explained by modified gravitation or other such effects.

Disclaimer: I do think that the Bullet Cluster is good evidence for dark matter, but we can speculate a bit

One is that there's also a possibility that dark matter exists but it's only part of the story.

For the Bullet Cluster I wonder how reliable the estimation of gravitational mass is for cluster of objects, as I understand, it is the images of the galaxies behind it that are lensed by mass in front of it https://en.wikipedia.org/wiki/MACS_J0025.4-1222#/media/File:...

For one heavy object in front of another, it's easy to see how lensing works and estimate masses from there.

But with clusters and "fuzzier" mass distributions I'm not so sure.

Thank you for posting this. I often read commenters who dismiss dark matter the way dark energy is dismissed without people addressing the bullet cluster style observations which in my layman view is the most compelling piece of evidence for dark matter

Is there a well defined boundary between QM and GR?

In most cases, only one of them maters and the other is negligible: at very small scales QM rules, while at very large scales only GR is important. But in special situations, such as black holes and big bangs, nothing can be neglected and there is no boundary. (AFAIU, IANAP)

What about in between small and large scales? Medium scales? Is there an in-between?

Yes, there is an in-between at which Newton laws hold. Too big for QM and too small for GR to make a difference. Fortunately, this is the scale in which we live.

Small point of clarification, the predictions of newtonian laws are a limit in GR. So, technically speaking, newtonian physics is still following GR for that energy regime.

So, QM at the small scale and GR everywhere else? Where is the line between QM and GR? Atomic level? Quark level?

QM has a classical limit too. At human scales, both QM and GR are at (very close to) their respective classical limits.

The Planck Scale gives typical dimensions (length, time, energy, mass) for a process where both GR and QM are relevant at the same time. Unfortunately, no experiment will get anywhere close in the foreseeable future.

I thought the Planck scale was way on the QM side of things. My understanding is that the Planck scale is so small we theoretically can never test QM or any other processes below it, making the Planck scale the limits to our understanding of the extremely small.

The plank scale is derived by saying "what length scale appears if we mash together the constants describing gravity and the constants describing QM". One definition of it is "what wavelength photons have enough energy to collapse on themselves and form a black hole". The only thing we know is that when an object is at that or smaller scale we need to consider both QM and GR. It is not "on the QM side of things", because making small wavelengths requires more energy, and more energy is more massive, and mass bends spacetime which requires GR.

and yet the universe is full of these objects - black holes, neutron stars, quasars, etc.

No, these objects have nothing to do with the Plank scale. They are described well by the QM and GR we already know and do not show any quantum gravity behavior.

Hawking radiation emitted by a black hole would be a quantum gravity effect. Unfortunately, it is predicted to be many orders of magnitude to weak to be measured.

I don't think your description is correct. The situation is we have a reasonably exact theory that assumes that most of the matter in the universe is neutral in the sense of only interacting gravitationally. But that interaction can be calculated quite readily in certain circumstances - for example, we can observe the "dark matter halo" in galaxies through gravitational lensing.

This is the dominant theory, it's not in contradiction with observations, it's just, a little weird. Why things are like that is the mystery but that's it for dark matter.

Sure, this stuff is invisible. So is electricity but we deal with that.

(Dark Energy I don't enough to talk about).

> Sure, this stuff is invisible. So is electricity but we deal with that.

Good description but bad example :). Photons are literally excitations of the electromagnetic field. You could say that everything except electricity [1] is invisible. Neutrinos are a better example as a known form of dark matter.

/Pedantic

[1] yes yes electricity is just one phenomenon of the electromagnetic field. I'm just differently wrong.

Many photons are invisible. The point is that not that dark matter is similar to other things in general, but that it's similar to a large list of things we take for granted in that you can't see it with the unaided eye and it would have sounded like woo a few hundred years ago.

Worse yet: electrons passing down into ground state will emit a photon. In essence, all we see ever is direct electrical activity.

That isn't a counter to the claim that electrical activity we don't see is something we take for granted that happens all the time around us.

I think it is a more than a little weird, as we find that over different scales our ability to predict loses its precision. We attribute this to an unknown something, that is sometimes referred to as "dark matter", but in reality is something we have not directly observed. Dark matter is essentially a placeholder for our ignorance, but with a little bit of assumption sauce thrown in.

> It would, I think, better inform the public about the actual process of science, rather than making it sound like a new discovery has been made, which has not happened, clearly.

I disagree with this -- dark matter and dark energy have been discovered in exactly the same sense e.g. quarks have been discovered. Like dark matter and dark energy, quarks cannot be observed directly. Rather, physicists have made observations that are most parsimoniously explained by the existence of a theoretical entity with certain characteristics we have named a "quark".

Similarly, physicists have made observations that are most parsimoniously explained by the existence of theoretical entities that behave like matter and energy but are otherwise difficult to observe (like quarks are difficult to observe directly).

The standard model provides a theoretical understanding of the nature of quarks -- but if further findings in particle physics disprove the standard model, then we'll have invalidated our understanding of the nature of quarks.

Similarly, if a future theory shows that dark matter and dark energy are not weakly interacting matter and energy then we'll have invalidated our understanding of the nature of dark matter and dark energy.

I disagree.

Dark matter is form of matter (matter is any substance that has rest mass) and it's dark (implying we can't see it and don't know what it is made of). Most hypotheses around the dark matter assume it's either new kinds of particles, or macroscopic objects.

Dark energy is also good name for hypothesis for unknown form of energy.

"Dark matter is a form of matter..." is a definitive statement, but you can't point to a single example of a direct detection of dark matter. It's quite possible the hypothesis is wrong.

The article, and the OPs aren't saying we shouldn't explore these hypotheses. They're just pointing out that they are, well, hypothetical. We should be cautious of talking about these things definitively as though we know they exist.

Hypothesis should be named correctly. If the hypothesis is it's matter, it should be called matter.

Primary evidence makes it look like it's matter even before any hypothesis.

Practically all hypotheses looking the evidence assume it is form of matter. Not calling it dark matter just because there is small change it would be something else would be misdirection.

> "Dark matter is a form of matter..." is a definitive statement, but you can't point to a single example of a direct detection of dark matter.

Are these not all cases of direct detection: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...

Direct detection would be detecting/trapping the dark matter particle(s). Those observations are indirect and it may turn out that there is another explanation other than dark matter such as we are using GR equations incorrectly as suggested in the post.

What kind of detecting/trapping? We have to be specific. If we hypothesize a kind of matter that does not interact electromagnetically (only gravitationally), then it's kind of nonsensical to say that only direct electromagnetic interactions with such matter will count to confirm its existence.

This is exactly why the hypothesis is weak. At the moment it postulates a particle that only ever manifests in such a way that it explains gravitational behaviour at a microscopic scale, in certain observations, and has no other detectable interactions or properties whatsoever. There’s no theory of how it forms, no theory of how it might be created or annihilated, no evidence for it in any particle physics experiments. We just have this one class of equations that produce unexpected results, so we put in an extra term and say were done. That’s just not good enough to say anything definitive about what it is.

Physicists know that’s not good enough, and that it’s a major problem. I hope we do make progress, do come up with further experimental evidence or ideas for experiments we can test. For dark matter I think that’s actually likely, but I’m not so convinced about dark energy, there the evidence is a lot thinner.

Well that is the crux isn't it? How do we directly detect gravitational particles?

Maybe we don't. The evidence for general relativity is all indirect--we can't directly detect space-time. But plenty of people seem to think it's a solid theory anyway.

There is a significant difference in that GR made numerous very testable claims about all sorts of phenomena, which would not be at all expected without GR to explain them.

It also depends what is consider direct detection, which I accept is a matter of opinion. For example I'd consider comparing the times on atomic clocks inside and outside a gravity well to be direct measurement of time dilation.

How do you distinguish direct as opposed to indirect? I mean, to me it directly observes something which is matter and non-luminous - and satisfies the technical definition of dark matter.

Rotation and velocity hint that something is amiss but doesn't directly measure dark matter. Gravitational lensing is a direct measurement of matter but doesn't tell us much about its composition. If using the most general definition of "dark matter", lensing would be a direct measurement. But if "dark matter" implies non-baryonic matter, the leading hypothesis, then gravitational lensing is indirect because it is hinting something is amiss just like the rotation and velocity evidence. Now if there was a predictive theory of non-baryonic matter and that predicted gravitational lensing with specific characteristics, that would be direct evidence. Just like GR predicted black holes and we searched and found them.

It doesn't appear to be dark, but transparent, AFAIU. For an object to be "dark", it has to be actually absorbing light, and I think dark matter doesn't actually absorb it, it just doesn't interact with it (except for gravitational lensing).

Or at least, that's what I've always understood. Not a physicist though.

I would say "dark" here is more like "unknown" than a color reference, while both obviously have a connection in how we experience.

> Dark matter is form of matter (matter is any substance that has rest mass)

There are many bosons that are massive (Z bosons for example) but are not considered matter. I think that normally fermions are considered matter.

Edit: s/barion/fermion/ bah, I know nothing.

Edit2: and of course gluons are not massive.

super pedantic and of course IANAP.

So, there is large quantity of matter permeating the universe with unknown properties? Doesn't that imply that our understanding, i.e., theories/models, are inconsistent with observation? All the the term 'dark matter' really implies is that there are observational anomalies caused by something that could be matter-like in its nature, but which we have not observed. It isn't necessarily at all like the matter we have observed, but could be matter-like. Or we could just be wrong in our models.

"Unknown form of energy" is not a hypothesis. It just means "we have a problem here; we don't have a solution, so we'll just rename the problem as a hypothesis". I think it's a clever sort of cheating (and you probably have to be very good at writing cosmology papers to get away with it).

Yes, interesting way to look at it. Calling it dark matter/energy also implies the issue is with the theory - we found something that the theory (or equations, as per the article) forgot to accurately account for.

It kind of implicitly throws out the notion that there could be an issue with the measurement process/tools.

Sometimes this type of unconscious framing directs all our minds down the same path making us miss an obvious detour to an insight.

> It kind of implicitly throws out the notion that there could be an issue with the measurement process/tools.

AFAIK this was the assumption early on, space dust, black holes/brown dwarfs (MACHOs), weakly interacting massive particles (WIMPs) and more were the leading theories. These are all normal (for physicists/astronomers) matter that just we couldn't see due to measurement limitations, so dark matter was an appropriate name. Only after all these theories were falsified we had to look at more exotic explanations.

Also obligatory xkcd: https://xkcd.com/1758/

WIMPs is still the leading theory right? The catch is that no candidate P has yet to be found and most models predicting them (Supersymmetry) have fallen out of fashion. But there are still promising and not yet completely discredited ones.

Yes, though there's a (minor, understood by everyone in the field) issue with the WIMP name: "Weakly Interacting" can lead to confusion that they must interact via the Weak force. They don't strictly need to: they could interact only gravitationally. That would make all the weak-force interaction experiments (big Xenon tanks & the like) fail, leaving only the gravitational evidence.

To me the concept of dark matter and dark energy reject issue with both the theory and measurements.

These concepts assume that both are correct, in which case the answer has to be matter and energy that is there but that we don't see.

The term 'dark matter' has something of an interesting history. Lord Kelvin (yes, that Kelvin) talked about 'dark bodies' in 1884 when discussing the missing matter of the galaxy. Henri Poincaré, writing in French, used the term 'matière obscure', which translates to 'dark matter' (kinda) while discussing Kelvin's work in 1906. In the late 1970's and early 80's, Vera Rubin, Kent Ford, and Ken Freeman gave the evidence that dark matter was a really real thing all over the universe and they used the term 'dark matter' in those papers to describe their measurements. In 1984, George Blumenthal, Sandy Faber, Joel Primack, and Martin Rees released a review paper on the recent findings of many groups that tied together Cold Dark Matter theory. I was a student of George, Sandy, and Joel's (in separate classes each) at UCSC.

Joel told us that 'dark matter' was a term used at the time and that coffee is a favorite beverage of many astronomers. As one of the local Santa Cruz coffee shops was serving a drink called 'double dark espresso' (caffeine levels fit only for a tired astronomer, I guess), he decided to name the ΛCDM theory after one of his favorite beverages, hence the 'Double Dark' Theory.

I think dark matter is a perfectly fine term. It's stuff that doesn't glow or interact with much of anything, but seems to just fall down. The term 'dark' is very much in keeping with the naming conventions of the post-war ear, where things are muted and humorously named (Big Bang, MAD, Charm quark, etc). The community has really embraced the term and, per at least Joel's theories, has remixed the term into coffee and other drinks, riffing and expanding the names to encompass other 'fun stuff'. I'd expect the names relating to various scotches to creep in soon (double cask, 15 year, etc), if my friends' predilections are any guide.

As a member of said general public I’m a little shocked by this revelation.

Thanks for the summary.

The inconsistencies are not named dark matter. But the leading explanation for the inconsistency is, quite abtly named: It's particles with a mass (=matter) which interact electromagnetically so weakly that they are non-visible (=dark).

Yes, the inconsistencies are not named thus, but the explanation for the inconsistencies are poorly communicated. Saying they are particles says too much, we don't yet know that there are other 'dark' particles either. We only know that our observations have an inconsistency, and we can 'explain' that inconsistency by positing that the observable/known matter is interacting with unknown/unobserved matter-like stuff. Giving this stuff a name like dark matter implies far more than we really know to the public.

I don't think so. The DM hypothesis is matter, not only matter-like, for whatever that is, which is dark. That's exactly what the hypothesis is. What do you think is implied which is not part of that hypothesis?

I understand one hypothesis is that it is some sort of matter, but the anomaly is that we can't fully account for why galaxies don't fly apart, given our current models and observables. One explanation for the anomaly is that there is stuff which interacts with observable matter in a matter-like way. That is, we don't know there is any new form of matter, rather, one of the conjectures is that the inconsistency we have observed can be explained by positing a new form of matter which interacts with known matter in a way we have not yet experienced or accounted for in previous models.

The point is, the real discovery is not the existence of any new form of matter, because we have not discovered a new form of matter, rather, the discovery is that we've found a very interesting point at which our theories are inconsistent with reality.

That's all more or less true. But what part of dark matter, the name of the conjecture, as you say, or the leading hypothesis, as I say, runs against this description? It does not imply that it's the only hypothesis, it does not imply that we discovered dark matter. Btw, stuff which interacts matter-like /is/ matter.

I've had a number of discussions with lay-folk who think we have in fact discovered a new/exotic form of matter. That is what gets lost. What we have discovered is that something went wrong in our predictions, which could be explained as a new form of matter (or more than one new form of matter), but, could also be explained by changing our models in any number of ways, etc. This is not at all obvious to the public.

I say "we" in the broadest sense of physicists, as I did study that for my grad/undergrad, but do not practice in the field (I now do the computering).

That may well be the case, but that is not caused by the term "dark matter", but by people not making enough of an effort to distinguish between leading hypothesis and discovery. That could happen with pretty much any name.

I'm pretty sure it wouldn't happen with the term Dark Hypothesis, or Dark Inconsistency. Or something cooler than those :^) . I think the words 'matter' and 'energy' are the confusion because they are already overloaded with meaning, even if not properly understood by the public, they still have some sort of idea of what those are.

It might be interesting if we had a standard, and publicly known, word or phrase used when talking about scientific results that find an inconsistency between reality and theory (not just slang). Like, "today physicists discovered a Theory Bug when applying Einstein's general relativity to galaxies" or some such. Something cooler than that though.

When we say inconsistency we normally mean a small difference; my understanding is that physics is missing 75%+ of the stuff of the universe that is required to make the sums add up. That's not an inconsistency, it's a dissonance.

It's only dissonance from the homocentric position of thinking we're the dominant form of matter in the universe. As it is all the parts of the universe relevant to us are tiny flecks of rounding error in the universe, now we seem to be finding the same with stars. Just like the familiar universe is mostly boring hydrogen and helium in stars, that's now being dwarfed by even more boring matter. It's like when we went from a geocentric view of the universe to a heliocentric one only to discover that the sun wasn't the center of the universe either.

A challenge to this line of thinking is the youth of the universe. Why is the universe ~14 billion years old, in the fleeting Stelliferous era, when we know that red dwarfs will last for ~10 trillion years?

Not sure it's a direct challenge though, it is an open question. Our interpretation of the age of the universe is based on matter interactions and measurements alone. It is an assumption that the big bang produced both matter and dark matter. What if only matter was produced in the big bang, and dark matter existed long before that?

The normal matter and the dark matter is distributed roughly the same way in space. For example, the dark matter associated with galaxies is in the same places as the galaxies. This strongly suggests that either the dark matter was produced more or less like normal matter in the Big Bang or that it somehow interacted very strongly with other species in the Big Bang.

I would be surprised if dark matter predated the Big Bang and survived it in and meaningful sense. I wonder if an theory in which dark matter is conserved during the very early universe is even consistent with observation.

That's very true, time is the one dimension that earth and maybe one day humanity make up a significant chunk of. But some possibilities are that we're one of the first intelligent species or the anthropic principal, were observing because we can and life later in the universe isn't possible.

I don't mind at all that matter isn't so important. I just mind that we can't account for most of the stuff that's out there. We have no knowledge of it; does that mean that science is a failure?

I agree, but for different reasons: I think the terms "dark matter" and "dark energy", like "black holes" and "wormholes", have assumed certain semantic qualities that are out of proportion to their physics meanings simply because they sound cool and edgy, like something from sci-fi.

The popularization of physics is a ultimately a significant source of funding for physics, along various vectors, and so terms like this are incredibly valuable to the field, regardless of how poorly descriptive they are.

In any case, physicists working in the field know what the terms mean: a huge bag of observations that are inconsistent with basic theories that we know to be incomplete.

Yeah, I don't think the physicists are confused about the terms, it's the use of the words in the popular lexicon that I think breeds confusion, and sometimes causes a missed opportunity for educating the public on how science works.

Right, but there wasn't a council meeting that voted to name these things. Names and notation arise organically. And the first thought of a physicist publishing a paper isn't (nor should it be), how will the general public misinterpret this?

This misconception can be explained in one sentence when somebody asks what exactly dark matter and energy actually are, and it doesn't really matter what people who don't care to ask think about the subject, because well, they don't care.

Yeah, I know. It's just a little frustrating, as I have had relatives be confused by this, thinking scientists have discovered new forms of matter, and have asked me questions about it - only to be disappointed.

I was disappointed with their disappointment mainly because to me the finding of so significant an inconsistency is magically interesting. To be fair, I have a physics degree, and one of the things I loved about learning this stuff was finding all the places that our understanding falls apart.

There’s definitely a very fine distinction between, “here are a bunch of observations that make it seem as if there is a bunch of weird matter that we can only detect by measuring its gravitational effects” and “there is literally a bunch of weird matter that we can only detect by measuring its gravitational effects”. One being a hypothesis to explain the other.

Agreed. But we have to keep in mind that this is how science works. For example, if were strict, we could not say "there is an exoplanet orbiting that star", we could only say "there are a bunch of observations that make it seem as if there is an exoplanet orbiting that star". And if you take this to the extreme, we do not know if the star is there. It is the same for laws: nobody has observed gravity or GR, just their effects. We can get philosophic and wonder if we are here...

At some point, we have to assume that observations correspond to an external reality or everything gets meta. While I agree that it may be convenient to make more clear what we know about dark matter, it is not easy to find the perfect name and, it seems to me, not a task for physicist, who just need a name for the terms in their equations.

I completely agree with you. But what if these terms we're giving names to are just flat out just nonsense in the equations? Just a term to assign to the error to make the numbers work out?

That is precisely the hypothesis of the article. It looks indeed like a plausible possibility. And a scary one, because we cannot just solve the equations with total precision. From my layman point of view, it looks like a very interesting but very difficult problem.

Yeah, to me having a more interesting discussion in a public forum (news shows, or other) would be about this sort of thing. Until we have dark matter in our hands or have reproduced this new weird matter in a lab, there still remains the possibility that we just got something wrong somewhere. Which to me is just as interesting. But exploring the possibilities and working through how theories could be amended vs amending our catalog of nature is the stuff I think our society could benefit from participating in.

'Dark Matter' is physicist code for 'gravitating stuff that we cannot see'.

'Dark Energy' is physicist code for 'anti-gravitating non-stuff that we cannot see'.

Interesting, the term dark is also problematic, first used when little is known about something. Later common usage shifts to something backwards or even non-reflective. See “dark ages” for another example.

“Dark matter” would be better described as “perfectly transparent and non-interacting gravitational mass”; and “dark energy” would be better described as “inherent tendency of empty space to expand”, but both are quite a mouthful.

Very interesting! Thanks for clearing up this misconception.

So what are all these "looking for dark matter" experiments actually doing?

They are looking for dark matter, obviously.

It is still the most parsimonious explanation that there is some kind of gravitationally interacting stuff out there that we just cannot detect with our current measurement methods. It is the simplest hypothesis that can explain all of the available observations without adding too many epicycles to our otherwise well tested theories.

Some experiments are literally looking for new types of matter that could play a role in interacting with observable/known matter than could potentially explain the overall observations. Some folks are also tinkering with the models/equations themselves to explore how to better model the universe. See Modified Newtonian Dynamics as an example.

The point the article is making is that we are inconsistent in our application of GR to large scale structures. Should we be surprised when the results we get are inconsistent?

That's like pushing new features rather than fixing bugs in software

So, yeah, I like this a lot. I have a degree in physics, but have been writing software for a very long time now. This resonates with me. Though in this case, the new feature may be dropped when it turns out it really doesn't work.

Fortunately, there are those who try to fix the bugs, see Modified Newtonian Dynamics as a Dark Matter alternative (as I mentioned elsewhere).

That comparison seems inadequate. Rather: that's like labeling bugs which you haven't found the origin of.

No, it is like triaging bugs by looking for the most plausible origins first.

Not saying anything about the dark matter, but dark energy is highly likely just the basic energy of the empty vacuum. The problem is the fact our equations don’t give the correct answer for it.

If this is a breakdown event like dark body or just a within system surprise event. In other words, Need a total new system or just refinement of observation or equitation?

>That isn't the case at all. We have equations we use to predict observations, and in certain cases, those observations are not quite matching what we predicted. There are a lot of reasons why this can happen, and using the equations poorly is but one of them.

This is absolutely correct. What is its clear (and as evidenced by some of the replies to your comment) is that many so-called "scientists" are filled with hubris and lacking intellectual curiosity. They believe (more or less) that we know how "things work" even though our commonly accepted formulas and equations about how "things work" don't add up. In their minds, its just a matter of discovering a still-hidden particle or detecting "dark matter" or "dark energy" that must exist, because they are certain that their understanding of the universe is how things must be. Unfortunately, this is the opposite of how science works. Real science is based on verifiable measurements and observations that can be replicated and verified. Its fine and dandy to come up with formulas and hypotheses based on incomplete data sets, based on how you think things might be, as long as you understand that you could very well be wrong - no matter how well a given hypothesis may seem to work for a selected, discrete set of measurements.

Yeah, basically that happens. I ran into this in school as well, as some professors accepted the current models as reality, which is what made me so sad in grad school when I realized how untrue this was.

I think of physics as a program. That program is built internally of the various models/equations, and that program takes a set of inputs, which are observables, and a time index. The output is the same set of observables, but with their values at the time index input. That is, physics essentially predicts (forward and backward) from a given set of observations, a new set of observations.

So to me, whenever the output observables do not accord with actual measurement, I first assume there must be something in the program that is wrong. Dark Matter is the equivalent of assuming that there must be another set of missing input observables into the physics program. It's possible, but to me the most likely first cause of the inconsistency is the program itself, as that is, as you said, how science works.

Unfortunately (as evidenced by the downvotes to the post you have replied to) many people have their entire careers and, more importantly, understanding of reality married to orthodoxy. In many ways this world view is like a religion. Dogma that has been accepted and internalized. Its sad and ironic that these people consider themselves to be scientist when, in actuality, their defense of dogma is the opposite of what science is all about.

I have this feeling as well, but now I realize that [citation needed]...

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