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.
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.
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?
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.
> 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?
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.
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.
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.
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.
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?
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.
[1] https://tritonstation.com/2026/10/15/ghostly-galaxies-in-the...
> 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.
“We don’t know what dark matter is or how to directly measure it but this distant galaxy has none”
Ok then
> 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 ordinary matter that gets tangled up is interstellar dust clouds, not stars and planets.
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.
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.
https://www.quantamagazine.org/triplet-of-ghostly-galaxies-h...
Thanks! Weird that the link didn't post.
Probably because you are posting in "Ask”.