Theories are not necessarily absolutely true. They are models that describe and help us understand problems. In most (all?) cases they are approximations. We know that there are problems with them but they are the best tool we have to understand something. Until another tool comes along that works better, the current tool will sufficient.
We know that General Relativity is incomplete. It cannot describe the work at high energies or very small scales. It is useful as far as it goes but it is not a perfect match.
Quantum Mechanics does describe the world of the small and the hot, but we cannot apply it to the larger world. QD, too, is incomplete. We look for a new theory that can combine the features of both but it evades us, so far.
Dark Matter is a similar theory. It describes a world. It seems to describe this work better than other theories like MOND. Looking for actual dark matter will let us see if DM is a good enough theory or not. If another theory can be found that is a better match, we can switch to that. In the meantime, every time we continue to try to verify/disprove DM.
We've been looking for dark matter for a long time without success. Until we actually find it, I think it is premature to dismiss out of hand other theories that explain the same phenomena. I'm not sure MOND is any better than DM, but I think it is worth at least pursuing the idea that maybe DM doesn't exist and there is something about gravity that isn't fully explained by general relativity.
> Until we actually find it, I think it is premature to dismiss out of hand other theories that explain the same phenomena. I'm not sure MOND is any better than DM, but I think it is worth at least pursuing the idea
“Dark matter” is just a phenomenological name to whatever mechanism that might explain all the observations consistently. Under this banner, people have studied models of everything from axions to small black holes. Speaking as someone who’s worked in the field, nobody is “ignoring” alternatives to dark matter — if they could think of ways to make up new alternatives (leveraging MOND, or otherwise) they totally would do it — the problem is that most such alternatives raise more complicated questions like breaking the consistency our theory of gravity, or introducing bizarre quantum behavior of certain cosmological fields, etc. Maybe all those problems can also be solved — but nobody knows how! That’s why it appears like they “ignore” it.
Further, any new theory going to supplant dark matter needs to explain not just galactic rotation curves, but also beat the Lambda CDM model in its ability to model/predict details of the cosmic microwave background radiation. AFAIK, MOND or whatever else is not even close, when you try to put all that together.
> the problem is that most such alternatives raise more complicated questions like breaking the consistency our theory of gravity, or introducing bizarre quantum behavior of certain cosmological fields, etc. Maybe all those problems can also be solved — but nobody knows how! That’s why it appears like they “ignore” it.
To me as a lay-man, this seems like you are indeed ignoring non-dark matter solutions, as dark matter allows you to add various magical elements to explain up whatever is missing in other models. This seems very un-scientific.
> dark matter allows you to add various magical elements to explain up whatever is missing in other models.
It’s not like physicists are looking for every little possibility to be fanciful. Here’s the irony — The “DAMA” experiment has been claiming to have discovered dark matter particles for years, but most physicists don’t trust that the experiment is understood well enough to call it a victory. The process of “adding particles” is an extremely rigorous one (the various detailed checks and stringent requirements involved are unsexy content ignored by pop science). Adding particles is how we discovered the Higgs boson, the weak nuclear force carriers and the internal structure of the proton. It is a battle tested process that underpins the tremendous success of particle physics in the twentieth century.
> To me as a lay-man, this seems like you are indeed ignoring non-dark matter solutions
Seems like there is a massive disconnect with how one actually “does research”. Let’s say I completely agree with you and want to come up with alternative models, and I’ve read the literature — but don’t know how to solve the hairy outstanding problems. What do I actually do when I get up and go to work tomorrow morning? :-P
To me as a lay-man, it is unclear what you expect them to do instead. Which part of "nobody has come up with a theory that explains nearly as much as DM can explain" constitutes "ignoring"?
>but I think it is worth at least pursuing the idea that maybe DM doesn't exist and there is something about gravity that isn't fully explained by general relativity.
I don't think anyone is disagreeing with this. However, the current observations are best explained by dark matter. The various MOND models don't fit as well. We either actually detect and confirm dark matter or someone comes up with a MOND model that fits as well or better and has testable predictions.
When people deride MOND, it's not to say that it can't be an answer later but people who are pushing it now are all ignoring some gaping hole in their theory.
Continuing research is fine but, confidence wise, dark matter is in the lead by a lot.
Dark matter ratios are not free parameters, because galaxies did not form through independent processes. The evidence for a highly homogeneous early universe is overwhelming, and any dark matter candidate that required large-scale inhomogeneity would be rejected out of hand.
If they aren't free parameters then why do we assign arbitrary dark matter content and distribution to galaxies like the bullet cluster/ultradiffuse galaxies with "too much/almost no/no DM" to "make it work out"?
If dark matter distribution weren't arbitrarily assignable, why would reputable physicists suggest that some galaxies might have a dark matter clump at the center and not a supermassive black hole?
The evidence for a highly homogeneous early universe has basically been refuted, unless systematic difficulties in assigning galaxy distances (certainly a possibility) JWST is observing galaxies that are forming WAY too early for a standard gravitational model; MOND has predicted early galaxy formation for more than a decade now.
> If they aren't free parameters then why do we assign arbitrary dark matter content and distribution to galaxies like the bullet cluster/ultradiffuse galaxies with "too much/almost no/no DM" to "make it work out"?
Something has to explain why galaxies aren't all exactly the same right? Whatever other theory out there will have some "free parameters" that explain that difference won't they? The observable matter differs, why shouldn't dark matter?
I think you're being really cynical about the scientific process here. The process is a new observation comes along that's a bit out of line with the norm and scientists look at the parameters and see if Dark Matter is still a candidate to explain it. Do you think it's unrealistic to assume that different galaxies might have different amounts of dark matter (should it exist)? They have different amounts of observable matter. If there was some galaxy that just didn't make sense by just adding some arbitrary amount of dark matter, it'd put a real damper on the theory but that hasn't happened yet. As of today, the only thing that seems "off" about galaxies is the amount of gravity which is very simply explained by adding in some amount dark matter.
I think my intuition of that is probably the same as your, that it seems kind of ridiculous to just add some invisible matter to the equation but it does actually do a really good job of explaining a few different phenomenon. I suspect the process for the acceptance of dark matter initially began as a "these galaxies are behaving _as if_ they had more matter" and a lot of people trying to explain why that might be to no avail until they kind of just accepted that maybe there just _is_ more matter that we don't see. At least that's how it's been for me. I'm still holding out some kind of hope/expectation that there's a better theory out there that actually explains dark matter/energy in a more satisfactory way.
It's not rediculous to add invisible matter. This is how we discovered Neptune, and Uranus. But it didn't work for Vulcan and Pluto was super sketchy. But to scientist's credit they gave up on dark matter Vulcan when nothing was found, over and over again, and GR explained mercury's orbit.
The analogy would be if we didn't figure out GR, used dark matter to explain mercury's precession, then used dark matter to explain aberrant voyager probe kinetics, then rejected GR because "it failed to explain the weird motion of voyager"
Adding invisible matter is not crazy, but over time you make your model increasingly unfalsifiable as the observations fail to meet the expectations. Ideally that would decrease confidence in the model, but for dark matter it seems to have entrenched the conviction of mainstream astro.
We know that General Relativity is incomplete. It cannot describe the work at high energies or very small scales. It is useful as far as it goes but it is not a perfect match.
Quantum Mechanics does describe the world of the small and the hot, but we cannot apply it to the larger world. QD, too, is incomplete. We look for a new theory that can combine the features of both but it evades us, so far.
Dark Matter is a similar theory. It describes a world. It seems to describe this work better than other theories like MOND. Looking for actual dark matter will let us see if DM is a good enough theory or not. If another theory can be found that is a better match, we can switch to that. In the meantime, every time we continue to try to verify/disprove DM.