Hacker News Reader

>But that's not important; the crucial point is that all of them can be _simultaneously_ fit perfectly by adding in the _same_ amount of dark matter.

Is that true? There are claims of galaxies with no dark matter, or tons of dark matter. It seems like dark matter is a parameterized value with lots of local anisotropy across the universe.

View on HN

It's not that the dark matter distribution is isotropic (it's not, as you note). Think of it like this: locally for a given object in the universe, we can work out the amount of dark matter necessary to describe one of its anomalous properties under the DM model. That same amount will work to explain the other anomalous properties locally for that object. You can do this at any point in the observable universe, including the unusual cases like the DM-deficient galaxies, and the model still holds up. MOND and related theories struggle in this regard, which is why DM is viewed as a more likely explanation (whatever the underlying nature of DM might be).

"MOND and related theories struggle in this regard"

It's often said that mind has difficulty explaining bizzare galaxies but my understanding is that's not true. I thought the only thing that MOND cannot explain is the matter distribution in the early universe, which requires an anisotropy that MOND does not "give for free" like dark matter does.

MOND famously (notoriously?) cannot explain the observed motions of galaxies within groups and clusters (the oldest known evidence for dark matter). Even if you assume MOND is true, the galaxies are moving too fast, so you need an extra source of, well, dark matter.

Reply on news.ycombinator.com