It bugs me that articles about dark matter so frequently frame things as dark matter vs. MOND, and don't consider any of the other theories of modified gravity. I think MOND is just not a great theory and it has a lot of evidence against it by this point; but other theories of modified gravity have not, to my knowledge, been tested to the same extent, nor do they get anywhere near the publicity. Dark matter still seems more likely than modified gravity, but I wish they wouldn't emphasize MOND so much as the representative of modified gravity, and I wish we'd hear more about tests of other modified gravity theories!
For someone like me who doesn't know much about modern physics, dark matter has the vibe of a band-aid to me. Like a programmer who discovered that the algorithm was always off by 1 in the results, and just ended up returning r + 1 without thinking twice.
I'd bet a large chocolate cake that one day, physicists will discover a much more logical explanation for this unexpected behavior of matter.
We noticed that some equations only make sense if mass is greater than what we'd expect from what we can see. So for now we pretend that there's this dark matter that we can't see, and with it all math so far checks out.
But we don't know if that missing mass exists at all, or if some formulas are wrong. So it's definitely a "hack", but it works for now and we don't have anything better. And we do expect to find an explanation at some point.
I'm not a cosmologist, but the Wikipedia article on dark matter has a pleasantly readable summary of the main sources of evidence for dark matter at this time.
Also see [1], a follow up post by Stacy McGaugh, a MOND proponent, who is quoted in the Quanta article. He does not agree with the evidence as presented in that article.
> DF2 has consistently been portrayed as fatal for MOND despite consistently being shown not to be a problem. I’ve given up writing formal papers showing this, as nobody cares. They just hear what they want to hear, which is that MOND is wrong. Indeed, after the 2018& incident I swore off doing so.
This should set off crank alarms when you read it.
Professor McGaugh has commented many times on the sociology of science and how the dominant paradigm tends to be hostile or indifferent to alternate views. He is also quite frank about the limitations of MOND as well, although he is definitely in the MOND camp overall.
He is hardly a crank - but he is definitely not in the mainstream camp.
perusing his blog posts, basically if a calculation is at all within expected values, this is evidence of MOND, but for something to be evidence of dark matter it has to jump through quite a few hoops in his opinion.
He also seems to have the same problem that many MOND proponents have which is arguing for MOND in blog posts instead of in papers, though he doesn't seem to be as bad as others in this regard.
I don’t really understand. Dark matter is in most galaxies interacting gravitationally, but not these three. Their idea is that somehow there was a collision event which stripped the galaxies of their dark matter, because the regular matter got tangled up but dark matter somehow didn’t.
But if dark matter is also interacting gravitationally, why wouldn’t it also be tangled up? And is that something we have seen in other collisions ever?
the Bullet Cluster makes for a good comparison, there's a good image of it in the article. the bulk of the 2 galaxy clusters continued on but left behind 2 clouds of inter-cluster gas
my first impression was this could actually be a counterexample, because there we can see the heavy, dark matter-full clusters continuing on their trajectories at full speed whereas the gas slowed down and is being left behind. compared to these 3 lightweight galaxies which seem to be alone, rather than trailing behind their dark matter-hosting parents
but perhaps these 3 _are_ going slower than their full-scale origin galaxies. I would stake my view as a layman on whether anyone can find the dark matter galaxies that spawned these farther along their expected trajectories
> If two typical galaxies met in a head-on collision, the dark matter would experience very little friction, or drag, and would keep on rocketing by. But ordinary matter, which interacts using all four fundamental forces, would get all tangled up.
Isn't a galaxy mostly empty space? I thought I read before that when galaxies collide there's actually very little physical contact between the matter. Stars mostly pass through affected predominantly only by change in surrounding gravity. Is this not the case?
The mean free path of a hydrogen molecule in the "warm interstellar medium" is about 0.03 light years, much smaller than the diameter of a galaxy. So if gas clouds collide they will interact strongly.
They don't collide but they do get "tangled" in each other's gravity. Small things (stars) get tossed in random directions and big things (gas clouds) pass into each other and start condensing into smaller/brighter things.
It can't be in each other's gravity, else the same phenomenon would affect dark matter. For something to affect regular matter but not dark matter, it has to be an electromagnetic interaction (at this scale - as the weak and strong forces only interact on very very small distances). So it has to be a significant amount of head-on collisions, or some kind of charged dust clouds projecting large EM fields.
I'd bet a large chocolate cake that one day, physicists will discover a much more logical explanation for this unexpected behavior of matter.
We noticed that some equations only make sense if mass is greater than what we'd expect from what we can see. So for now we pretend that there's this dark matter that we can't see, and with it all math so far checks out.
But we don't know if that missing mass exists at all, or if some formulas are wrong. So it's definitely a "hack", but it works for now and we don't have anything better. And we do expect to find an explanation at some point.
[1] https://tritonstation.com/2026/10/15/ghostly-galaxies-in-the...
Ok then
This should set off crank alarms when you read it.
He is hardly a crank - but he is definitely not in the mainstream camp.
He also seems to have the same problem that many MOND proponents have which is arguing for MOND in blog posts instead of in papers, though he doesn't seem to be as bad as others in this regard.
But if dark matter is also interacting gravitationally, why wouldn’t it also be tangled up? And is that something we have seen in other collisions ever?
my first impression was this could actually be a counterexample, because there we can see the heavy, dark matter-full clusters continuing on their trajectories at full speed whereas the gas slowed down and is being left behind. compared to these 3 lightweight galaxies which seem to be alone, rather than trailing behind their dark matter-hosting parents
but perhaps these 3 _are_ going slower than their full-scale origin galaxies. I would stake my view as a layman on whether anyone can find the dark matter galaxies that spawned these farther along their expected trajectories
Isn't a galaxy mostly empty space? I thought I read before that when galaxies collide there's actually very little physical contact between the matter. Stars mostly pass through affected predominantly only by change in surrounding gravity. Is this not the case?