[Pxtl]

There's a good cosmology YouTuber who basically explains: dark matter is not a theory, it's a set of observations. Galaxies behave like there's a halo of invisible mass surrounding them, one that varies in density per-galaxy. The halos even perturb as expected in galaxy collisions. But nobody's got a coherent theory on what that halo is.

https://m.youtube.com/watch?v=PbmJkMhmrVI&pp=ygUbZGFyayBtYXR...

[nimish]

This is maybe half true. Dark matter is required to explain these under the current lambda-cdm model, but the behavior of galaxies is not evidence for it in a constructive sense. It's only evidence that the current model is untenable.

Galaxy rotation curves are better explained by applying general relativity without the severely restrictive assumptions required in lambda cdm. You don't even need the full thing, just the first order linear approximation that allows for gravitational waves (and thus is causal), as Ludwig showed a few years ago. You need at least this because gravitational waves exist, and those cannot occur in the singular newtonian limit used mostly for convenience.

It doesn't take much to then question the need for dark matter as if it is compensating for poor models in one case, it probably is doing the same in others.

[naasking]

> Galaxies behave like there's a halo of invisible mass surrounding them,

No they don't. Galaxies exhibit anomalous velocity curves according to existing gravitational theory as applied in a specific cosmological model. That's the observation. This could be explained by a halo of indivisible stuff or it could be explained by a misunderstanding of gravity or a different cosmological model.

I'm general, we should not explain observations in terms of speculative theories, because this carries a presumptive bias.

> The halos even perturb as expected in galaxy collisions.

They actually don't. Dark matter would explain part of the bullet cluster (lensing), but it can't explain the high collision velocities observed.

[aeturnum]

Haha - I simultaneously wrote my own comment that recommended the same video! I think acollierastro makes really good, accessible physics content.