It's a common pattern with crackpots as well, that they focus on "big" but easy to understand things like special relativity, which doesn't involve more math than you learn in high school. No crackpot ever revolutionizes condensed matter physics or statistical mechanics.
Although the bullet cluster should be a clear indicator as well, the rest of the evidence of DM appears much more subtle to a layperson.
Anyway, cosmology is a weird topic which not only HN reveals tons of arrogance over. Lots of misplaced skepticism. (Probably similar to the Covid topic.)
Well, the memory of water still has a lot of support, as the scientific explanation for homeopathy. Lots and lots of crackpots here.
I would not call this crackpot, but it took an awful amount of time for some to let go cold fusion.
As for statistics - my personal hell is when I have to read some biology or biology-like document and there are numbers that are tortured to make them sing the way they are expected to. I do not even talk about economy, psychology, sociology and other similar ologies.
It's the curse of being clever. People believe that since they're clever and were able to be successful at what they do, it must be that all other topics are similarly easy. And with easy access (via the internet) to the most basic introductions for lay people to any topic, the Dunning Kruger effect really kicks in.
The more clever you are, the easier it is to fall into this trap. example: "Buy twitter and solve all its problems myself".
There are infinitely many mathematical systems that could describe "reality" all with divergent behavior in the parts of the system we haven't observed yet. Trying to make authoritative claims by analogy to coincidentally correspondent mathematical systems is foolhardy and vain.
We have crackpots proposing that the Earth is flat.
These people are effectively asking about these objections. Perhaps many of them are phrasing it differently. But, with exceptions, I think people tend not to be angry or vitriolic. At best they are frustrated. Just treat these remarks as questions.
It might be annoying to get the same questions repeatedly, but that is a fault of wider communication about dark-matter.
Everything we see in the Universe outside our own galaxy is static. We don't have enough parallax to actually perceive objects (stars) moving outside our galaxy. Ok, there's an exception, people were able to perceive the movement of one star in one of our satellite galaxies in 2012. That's about it. So, when people talk about galaxy rotation curves, that's based on a sample of one, our own galaxy. Actually, we can plot galaxy rotation curves for other galaxies, but based on indirect evidence, such as the redshift. How accurate is that? Come to think of it, we can't estimate directly the distance to any galaxy beyond and including the Andromeda galaxy. But again, we can estimate indirectly. Only that estimation is based on a stack of estimations each with its own uncertainties.
Now, if you work in the area, stop me right here and tell me you went through that stack of estimates, and check all the uncertainties, and are confident there's no potential error anywhere.
Because, as an outsider, if I do a bit of Bayesian estimation, and I have to choose between the dark matter and the possibility that everything that can be explained with dark matter is actually a result of estimation errors, I'm somewhat on the fence. Every year that the hunt for dark matter brings nothing I'm more inclined to think there's a different explanation out there. Like the frame dragging explanation discussed in [1].
[1] https://physics.stackexchange.com/questions/618981/has-frame...
I'm not a cosmologist, but I have a degree in Physics. I won't be able to fill in the details, but all your objections sound pretty basic and unfounded even to my semi-amateur ears. About the cosmic distance ladder (as if parallax were the only way. I guess we don't see Earth's curvature either), about how to propagate errors (literally learnt in the 1st week of Physics. Those bars around the scatter points are literally the error bars, and we propagate them from start to finish). And even about the scientific method itself. No one can claim "there's no potential error anywhere" in anything.
And most importantly, which was my original main point, galaxy rotation curves are not our only evidence.
BTW, I would encourage you to look at how fossils are dated. To me it sounds kind of similar in spirit to how distances are measured in astronomical scales.
For example, here's a quote from the book "An Introduction to Modern Cosmology" by Andrew Little (third edition, published in 2015, page 70):
It is just about possible given present observations that this matter can be entirely baryonic, since this is marginally consistent with Equation (9.3). However, many models based on low-mass stars and/or brown dwarfs have been excluded adn it is probably difficult to make up all of the halo with them. A popular alternative is to suggest that this density is in some new form of matter, which is non-baryonic and only interacts extremely weakly with conventional matter.
Again, think of your area of expertise and apply Bayesian thinking to an amateur coming from outside and claiming things like you're doing here.
But this is a bit orthogonal. That was not the gist of my argument.
My argument was that lots of things in astrophysics are based on models. The rotation curves themselves, as a sibling comment mentions, are fairly directly observable, via redshift. But that's not fully true. Doing spectral observations is not entirely trivial, see for example [2]. Take a look at the sample profiles [3] and see how noisy they are. Of course, you can remove all sorts of noise with enough observations, and since (as I said before) the universe is static, and not going anywhere, you can point your telescope at the same galaxy and take lots and lots of pictures and do lots and lots of long slit spectroscopy from all sorts of angles, until you are left with very little noise (aside from the "thermal" noise due to the stars having some random velocity component).
How difficult, or easy is the math of eliminating that noise. Most likely it's not super difficult. Not trivial either. Probably somewhat similar to the math used for doing computerized tomography.
But here's the crucial difference. Between CT scans and astrophysics. And between my area of expertise that you encouraged me to consider and astrophysics. If the math for CT scans is wrong, people die. If the math I do for a living is wrong, people lose money (and I lose my job). This math with immediate consequences is self-correcting. If the math to denoise the galaxy rotation curves turns out to be wrong, someone just publishes a paper, and then gets to do a lot of talks at a lot of conferences.
Let's move on to the other evidence. For example the relative abundance of hydrogen, deuterium, helium, lithium. That is based on some models of nuclear interactions. Which agree with our experiments (in particular the crucial part is that neutrons have a half life of 600 seconds, and not, say 100 seconds). But nucleosynthesis happened only for the first 3 minutes after the big bang or so. Can we simply assume that general relativity effects were negligible? Let's put it differently: the whole universe was like a neutron star. Astrophysicists state that modeling neutral stars is difficult because quantum chromodynamics and general relativity happen at the same time, and we have nothing similar that we can study. But then how can we be so confident in our modeling of the early seconds of the universe, when, if anything, conditions were even more extreme.
The bullet cluster. I'm not going to point you to Sabine's blog post of how the bullet cluster is, if anything, a proof against dark matter. You can find it elsewhere in this thread.
Gravitational lensing. That's an impressive thing, but the claim that there can't be small black holes because we'd be able to detect them via gravitation lensing sounds to me quite silly. The space is just too large.
Now take our interstellar visitor Oumaouma. It just zipped by. Can the dark matter be just a large collection of such asteroids? Maybe not. But maybe at least a part of it can be explained by it.
Galaxy-wide lensing. The claim is that the effect of lensing is consitent with the mass of a galaxy being much higher than the one explained by its luminosity. But we know that all galaxies have a giant black hole in the middle. Can that black hole be bigger than we thought? No, you say, because of the rotation curve. But what if the rotation curve is erroneous?
Back to the rotation curve. Is there actually such a thing? Are the stars in the spirals moving at the same angular velocity as their cousins between the spirals? Most likely not. Is it possible that the galaxy rotation curve is a result of the non-homogeneity of the galaxy and the finite speed of the propagation of gravity (the speed of light) ? Here's how this would go: the attraction a start feels towards the center of the galaxy is governed not by what is between the star and the center now, but what it was a few thousand of years ago, when the spiral was a bit back, and so more mass was there. Conversely, the gravitational attraction away is governed by what it was there thousands of years back, when fewer starts were there.
You could say astrophysicists have thought of all these things, and about a thousand more. But do you see how everything depends on models? Models that can't be easily validated.
Is it plausible that there is dark matter? Absolutely. I don't see anything outrageous around the proposals that there are particles out there that interact only via gravity. But is it possible that there could be a different explanation that to various questions that does not require dark matter? That does not sound all that absurd to me.
[1] https://en.wikipedia.org/wiki/Cosmic_distance_ladder
[2] https://en.wikipedia.org/wiki/Long-slit_spectroscopy
[3] https://en.wikipedia.org/wiki/Long-slit_spectroscopy#/media/...
In their textbook you'll find that galaxy rotation curve studies are spectroscopic. That is they are keenly interested in the relative redshift in the 21 cm neutral atomic hydrogen lines (among others like H2 and CO molecular lines) at the limbs of edge-on disc galaxies, and adapting that to spirals and other disc galaxies that are tilted away from edge on. [Binney 2e sec 6.1]. Practically invariably the relevant lines are relatively redshifted and relatively blueshifted at the limbs, leading to the interpretation that generically there is equatorial spin in disc galaxies.
In elliptical galaxies there is practically no equatorial spin to speak of; instead the spread of relative redshift across the face of the elliptical is interpreted as blobs of hydrogen gas moving radially, that is sinking deeper into the galaxy or rising out of the galaxy's depths. It is also useful to study a wide range of absorption lines given that the clouds are backlit by a galaxy's worth of starlight (and sometimes a quasar), in a process which grinds out surface densities.
These relative redshifts do not depend on cosmological redshift (the whole galaxy, or its whole cluster, is cosmologically redshifted identically for all practical purposes, so the opposite limbs in discs are affected similarly). It is also not sensitive to an isolated galaxy's peculiar motion within a cluster. It may matter for merging galaxies.
We can also look deeper than disc limbs and ellptical surfaces. Optical interferometry is highly sensitive in this application, and provides direct evidence of the motion of the various sources of emission and absorption lines from various gas clouds, dusts, and even starlight. The Large Binocular Telescope does some work in this area. Radio interferometry is useful for looking into the bulk motions within the more central regions of galaxies and clusters; dust obscures optical signals but millimetre signals cut through.
The investigated starlight is bulk and while the interpretation depends on assumptions about the bulk stellar chemistry of an observational target (maybe metallicity varies slightly in different parts of an elliptical which might have a history of galaxy mergers) there is in no way a dependence upon any single star and its meanderings through its galaxy. We're interested in the light generated by ~billions of stars, not the positions or momenta of single stars, mostly because we just cannot resolve the latter with current technology.
> as an outsider
You seem interested in the topic. Learning how observations are made (and the history of them) is probably not inaccessible to you given your comments here. Whether that leads you into any sort of conclusions about the structure and evolution of galaxies is up to you, but I think textbooks will be better for you than whatever ultimately led you to the stackexchange link in your comment. (I did notice however that the Ciotti preprint discussed later in your link cites Binney & Tremaine multiple times, as does the Ludwig paper in the stackexchange question).
Okay now do climate change and climatologists.
And most people just don't bother to read the article at all it seems. No better than reddit.
Changing gears, though, and trying to be more useful, if you want more coverage on this topic there's a note in Peter Woit's blog:
https://www.math.columbia.edu/~woit/wordpress/?p=13232
Points to another blog which attacks the "nightmare scenario":
https://tritonstation.com/2022/11/29/the-angel-particle/
I don't particularly buy that a gravity-only interacting particle "is not physics". If that is what exists, that is what exists. And we've already detected it via dark matter observations. So it is physics. What we can't do is ever really prove it other than by exclusion, but if we racked up a thousand years of failure, we'd have to accept that it was real (hopefully by then not dogmatically and we'd let any kid who wanted to tilt at windmill to go for it).
And Woit links to a paper on dark matter being only a right handed majorana neutrino, requiring no real physics beyond the standard model (plus three right handed non-interacting neutrinos): https://arxiv.org/abs/1803.08930
And links to a comment on that in John Baez's blog:
https://golem.ph.utexas.edu/category/2022/12/neutrino_dark_m...
http://backreaction.blogspot.com/2017/01/the-bullet-cluster-...
Another big problem is that all LCDM predictions (MACHO/WIMP) were disproven but it still remains a theory. Now we need energy too high to use in universe.
It's eerily similar to Phlogiston theory that at one point was considered to have negative mass.
(One might remark that I'm just not intelligent enough to understand advanced mathematics, and they might be right...)