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It seems to me, as a layperson, if MOND was hypothesized first, it would be the dominant theory. It’s so elegant and simple with so much explanatory power. However, because it was hypothesized much later, the fact it matches past predictions is given no weight, despite its simplicity.

I think this is a bit of a mistake. I believe that simplicity itself is evidence. Dark matter theory is starting to feel like the epicycles of Copernicus.

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If MOND completely described observable phenomenon it would supplant dark matter, but it doesn't. MOND still requires dark matter, just less of it. Or, to quote Milgrom, "some standard matter in some form that has not been detected"[1]. Which, IMHO, is nearly isomorphic to DM as far as statements go.

So until we figure out what the missing mass is, MOND is widely viewed as an unnecessary complication. There is missing mass, if we figure out the nature of DM or discover this hitherto unknown "standard matter" then we can talk about modifying universal gravitation.

[1]: https://web.archive.org/web/20160721044735/http://www.astro....

The explanatory power of dark matter is so weak though. With the right clumping of LCDM you could probably also explain various reports of poltergeists around the world.

Then you could claim that reports of poltergeists are evidence of LCDM over MOND because it's another phenomenon that MOND can't explain.

However, there is plenty of evidence to suggest that it does exist.

The distribution of dark matter being uneven is evidence that something functioning like dark matter has to exist as MOND doesn't easily replicate that. Colliding galaxy clusters show a distribution of matter that differs from the distribution of visible matter, so the simplest explanation is that there is invisible matter that reacts gravitationally.

The simplest explanation is that there is regular matter which we fail to detect. Not weird, given the huge distances involved - there almost assuredly is mass that we didn't detect (e.g. planets).

The point where fairy matter comes in, is that we can guess how bad we could be at not detecting matter. Basically, regular matter that we didn't observe can't explain everything - we would've observed it otherwise.

Sorry, There are currently hard constraints on the lumpiness and size on that matter which basically exclude all reasonable possibilities for "normal matter we fail to detect". It's certainly possible something strange is happening though, or our statistics could be wrong.

That's not the simplest explanation as our current theories limit the amount of normal matter that can exist and galaxies behave gravitationally as though there must be a lot more than normal matter. Normal matter that is invisible to us is not a viable solution to the evidence.

There some discussion of regular (baryonic) matter vs dark matter here: https://www.livescience.com/how-much-dark-matter-universe

> The simplest explanation is that there is regular matter which we fail to detect. Not weird, given the huge distances involved - there almost assuredly is mass that we didn't detect (e.g. planets).

What is the difference between that and dark matter?

> there almost assuredly is mass that we didn't detect (e.g. planets).

Don’t planets give off light and thus contribute to the distribution of light observed?

The GP is talking about MACHOs; MAssive Compact Halo Objects. They used to be a valid DM candidate but have been ruled out 20-30 years ago. Planets don't give off any non-negligible amount of light. However, they do consist of baryons (read: atoms) which would contribute to the baryonic accoustic oscillations in the CMB power spectrum, so that's a good probe to set upper limits on baryonic matter in general.

Primordial black holes are a somewhat related candidate; but those are mostly considered ruled out as well. They are not considered baryonic matter.

The evidence that it exists boils down to: gravitational anomalies. It's not multimodal, which is a problem because you're saying "it must exist because of x therefore it must exist" versus "it must exist because of x, which has a side effect of y, which aha also we see". That's why it's weak.

> the simplest explanation is that there is invisible matter that reacts gravitationally.

That's the whole point of the poltergeist example. If someone said: "Oh, that urn on your mantle just fell over? Must have gotten knocked over by a rogue ball of dark matter. The simplest explanation is that dark matter must exist". You would think they are insane.

You can think of dark matter as gravitational anomalies that aren't distributed uniformly. If we assume that physical laws are constant throughout the universe, then it becomes difficult to account for those anomalies without saying that there is "something" there that is influencing gravity. Obviously, we've labelled that "something" as dark matter and figured out what properties it can and can't have.

With your poltergeist example - if it was relatively common for urns on mantle to be knocked over, then it would make sense to come up with a hypothesis. If objects around the urn were affected gravitationally just before the urn falls over, then dark matter would appear to be a rational hypothesis.

Problem that I see with dark matter is that it is unfalsifiable.

If you can have arbitrary amounts of dark matter in arbitrary positions - how can you falsify that?

This argument seems to pop up on HN every time dark matter is discussed. Unfortunately, it stems from a fundamental misunderstanding of how modern cosmology works.

The exact spatial DM distribution is not a parameter of the cosmological standard model. Instead, you assume an initial condition (pretty much a smooth distribution with only quantum fluctuations, which are parameterized by one or two parameters), apply the laws of physics to evolve this state some 14 billion years and compare the statistics of the theoretical and observed distribution.

In fact, DM suffers from the dwarf galaxy problem. Our theory predicts more than we observe. It's a bit of a challenge for DM, and considering how fuzzy our knowledge of galaxy formation is and how much other evidence for DM there is, it's not falsifying it yet. But if this is cemented by future observations it might very well be a blow to DM.

Besides, DM was hypothesized after measuring the famous rotation curves of galaxies. DM could have been easily falsified if we made gravitational lensing measurements afterwards and didn't see any DM. But we saw DM. Same thing with baryionic accoustic oscillations in the cosmic microwave background angular power spectrum. We looked there after we already thought that DM should be there, and the power spectrum looks just like if there is DM. It could have easily falsified DM, but didn't. The list goes on. DM could have been falsified may times, but passed almost all tests so far. Well, except for the ones in the lab, unfortunately.

It's not as if dark matter doesn't follow any rules at all - hypothetically, it obeys gravity like any other matter. So you can't have arbitrary amounts in arbitrary positions - you have to explain what attracted the dark matter to that position.

MOND mispredicts many, many past discoveries. This would have people scrambling for alternative explanations immediately. Dark matter is simply a better fit for what we see.

Second, I don't know why MOND seems simpler and more elegant to you than literally something that doesn't modify the laws of physics at all but just increases the mass you use in computations.

I’m not a MOND proponent, but Cold Dark Matter also literally requires modifying the laws of physics, by positing new particles.

I was pretty convinced that dark matter is made up nonsense and that RelMOND is probably correct, but then I learned of the Bullet cluster.

https://en.wikipedia.org/wiki/Bullet_Cluster#Significance_to...

Two clusters collided, with most of the observable objects being just past the middle point right now, however most of the mass seems have already shot way out to the edges, as if it passed right through everything. It's pretty hard to explain with anything else other than that there's something there that has mass yet can't interact with anything else that passed through everything when the collision happened. Adjusting equations doesn't really work in this case, there must be something physically there.

I'm not sure why this isn't mentioned first thing every time in dark matter discussions because it's literally the only thing that's pretty hard evidence imo.

> however most of the mass seems have already shot way out to the edges, as if it passed right through everything. It's pretty hard to explain with anything else other than that there's something there that has mass yet can't interact with anything else that passed through everything when the collision happened.

It is my understanding that dark matter does interact gravitationally, so I am confused by this.

I should've been clearer there, "has mass" would mean that it interacts gravitationally and has inertia while moving. But that's about it, from what I understand.

Here's a good response to MOND re: the bullet cluster - https://web.archive.org/web/20160721044735/http://www.astro....

I am too ignorant to weigh in on MOND vs dark matter, but this seems entirely possible.

In all domains - not only physics - there are many possible models that seem to match our observations. They’re continually adapted over time in order to meet to perceived needs and reality (see: the medieval European church).

Paradigm shifts can happen incremental or explosively. With the enormous sunk cost in our existing models - not least the millions of people educated in them - incremental change becomes harder. If MOND or any alternative theory is “true”, it faces a distinct challenge: there are decades of experiments and observations that have been fit to our existing model. To take over, it needs to answer _all_ of it - or provide so much utility that the incentives shift.

Success can be paralysing. In tech, a new idea can come along and shake everything up by showing results and solving a problem. The only way a new model of physics can do that is by generating a practical breakthrough. I hope I’m wrong!

Epicycles is what Copernicus fought against. Have he also have some I am not aware of ?!

Epicircle come about if one consider earth is the Center and some planets sometimes move backwards and then start to move forward again. You do not have that issues with Copernicus.

But he made a mistake of using circle. The observation guy using everything rotate around the sun and sun around the earth … a wrong model but the key is not the model but the observation he made. His record keeper Kepler finally found some other patterns and Newton get the … the rest is history.

I do not go to wiki or internet to check out the phrase. But epicircle …not of Copernicus.

Not an historian or astronomer but I guess Copernicus' main work is about heliocentrism, but it uses epicycles.

It has to because in his views planets made circles around the sun and at constant speed.

Indeed Copernicus was aware this only could not explain what is observed so he had to introduce epicycles as in Hipparchian, Ptolemaic system.

Circles with epicycles are just an approximation for ellipses, but ellipses could not be introduced without a theory of gravity.

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