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 have yet to see a model of galaxy rotation that doesn't use the shell theorem, which does not apply to disks. I've seen it in very advanced (over my head) math stuff on Wikipedia.
Im half convinced its gonna be a stupid mistake like that. Whoever finds the correct "advanced" math that allows the mistake to be brushed under the rug could get a Nobel prize.
The fun question to ask is why almost all galaxies (no matter how they differ) spin as though a simple modification of gravity explains them. That dark matter is organising itself to produce the same outcome over many different circumstances!
Do all galaxies spin in the same way? Do we predict an equal proportion of dark matter in all galaxies based on observations? Or is there some variability?
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 think its standing is a bit better than that. We have several independent lines of evidence like patterns in the CMB that aren't just "hmm, we need m to be bigger in our model of a galaxy." And we have a couple of energy scales compatible with standard particle physics where the dark matter might be hiding.
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.
Well, any other theory would have to explain all of the following better than cold dark matter does:
1. Galactic rotation curves (galaxies spin too fast)
2. Galactic gravitional lensing (galaxies bend too much light)
3. The shape of the cosmic microwave background (it only matches if you have dark matter)
4. The bullet cluster (visible matter collided, but the gravitional lensing kept going)
Non-electromagnetically interacting matter solves all of these, and the same ratio of matter:dark matter solves all of them. That would be one hell of a coincidence.
I don't understand why it has to be non-electromagnetically interacting. Can we honestly say we should be able to resolve all the matter in a galaxy with our current sensing technologies? Why can't it just be the equivalent of non-light-emitting asteroids and planets. We can barely detect whether a star has planets in our own galaxy. And according to Wikipedia the amount of matter we would be missing in a sphere with radius from the sun to Neptune is 10^17 kg or approximately a large asteroid. Which seems like a rounding error. But my knowledge of astronomy is definitely minimal.
The thing is, we've looked for them. If they were small (asteroid sized), the amount required would be visible as dark clouds obscuring the objects behind them. If they were large (brown dwarf or bigger), we'd see gravitational lensing from them. We don't really see either of those on the scale required to explain the gravitational effect.
Idea for a science fiction short story: dark matter interacts with normal matter only in specific conditions requiring advanced science, with extremely rare occurrences naturally.
Humanity, after migrating from biological carbon based substrate, finds ordinary matter not practical to live in. Dark matter has much better properties.
All humanity transfers to dark matter where it finds all the other "ascended" species historically hosted by our universe. Solves Fermi's paradox and keeps the "normal matter" nursery clean for new species to appear.
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.
According to MOND theorists, a dwarf galaxy like DF9 doesn't prove dark matter, since it's sitting in the gravitational field of a big neighboring galaxy (NGC 1052) and thus its boost is suppressed. They call this the "external field effect".
> 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.
> 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
Maybe it should - but it does not prove that the subject is a crank. Thomas Khun's theory of paradigms shows that science is not an entirely objective undertaking.
Having read Prof McGaugh's blog and some of his published papers for quite some time, I do not place him in the crank bucket. Make your own mind up I guess. My take is he is a legitimate scientist who holds views outside the mainstream.
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.
He is a professional scientist with a blog - he has written a lot of papers as well [1]. It is worth taking his views seriously, even if you have reason to dispute them.
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
> But if dark matter is also interacting gravitationally, why wouldn’t it also be tangled up?
Friction between regular matter causes it to lose energy and thus velocity. Dark matter doesn't have self-interaction outside of gravity, so it just keeps going.
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!
> 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.
Im half convinced its gonna be a stupid mistake like that. Whoever finds the correct "advanced" math that allows the mistake to be brushed under the rug could get a Nobel prize.
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. Galactic rotation curves (galaxies spin too fast)
2. Galactic gravitional lensing (galaxies bend too much light)
3. The shape of the cosmic microwave background (it only matches if you have dark matter)
4. The bullet cluster (visible matter collided, but the gravitional lensing kept going)
Non-electromagnetically interacting matter solves all of these, and the same ratio of matter:dark matter solves all of them. That would be one hell of a coincidence.
You are describing MACHOS, which was one hypothesis for dark matter. https://en.wikipedia.org/wiki/Massive_compact_halo_object
The thing is, we've looked for them. If they were small (asteroid sized), the amount required would be visible as dark clouds obscuring the objects behind them. If they were large (brown dwarf or bigger), we'd see gravitational lensing from them. We don't really see either of those on the scale required to explain the gravitational effect.
Because we have zero evidence of it electromagnetically interacting. We have evidence of other particles like this, like neutrinos.
> Why can't it just be the equivalent of non-light-emitting asteroids and planets
Because they've been ruled out: https://en.wikipedia.org/wiki/Massive_compact_halo_object
You are not proposing anything new. Your ideas have been thought of before, and discarded because they lack evidence.
Humanity, after migrating from biological carbon based substrate, finds ordinary matter not practical to live in. Dark matter has much better properties.
All humanity transfers to dark matter where it finds all the other "ascended" species historically hosted by our universe. Solves Fermi's paradox and keeps the "normal matter" nursery clean for new species to appear.
[1] https://tritonstation.com/2026/10/15/ghostly-galaxies-in-the...
https://arxiv.org/abs/2104.13961
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.
This should also set off crank alarms.
Having read Prof McGaugh's blog and some of his published papers for quite some time, I do not place him in the crank bucket. Make your own mind up I guess. My take is he is a legitimate scientist who holds views outside the mainstream.
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.
[1] https://scholar.google.com/citations?user=lM8J8noAAAAJ&hl=en
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
Friction between regular matter causes it to lose energy and thus velocity. Dark matter doesn't have self-interaction outside of gravity, so it just keeps going.
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?