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Comment by chongli | Hacker News Reader

That’s because dark matter is frequently referred to as a theory. It’s not. Dark matter is a set of observations which we try to account for in our theories. New theories which account for some observations of dark matter but ignore others are rightfully considered suspect.

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I'm not really sure that's a useful way of putting it. There are a set of observations which don't match expectations, one idea which explains most of them is that there is some matter present that we can't directly observe (and then there's a wide variety of more specific ideas of what the nature of this matter may be, some of which fit observations better than others). That's in contrast to the idea that our theory of gravity may be incorrect, which also explains some of the observations (including ones which dark matter does not), but less of them overall, and there's a wide variety of specific ideas of how we might modify gravity to fit the observations (and generally I think the difficulty is you can come up with modifications that explain one obversation well but it's hard to explain a lot of them at once).

DM and MOND are referring to different ideas of matching the anomolous observations, not the observations themselves. And each is really a family of different theories, as opposed to one singular theory each.

That’s not quite right. The theory which MOND is an alternative to is General Relativity. Dark Matter is the name given to the missing mass resulting from our observations (light we observe directly) not according with mass we infer to exist by GR and the amount of gravitational lensing we observe.

As an aside, MOND is not doing so well these days. One of the leading proponents of MOND has published a paper showing a need for substantial revision to the theory following observations of wide binary stars [1].

[1] https://academic.oup.com/mnras/article/527/3/4573/7342478

That's not quite right either. MOND is not a relativistic theory. The clue is in the name: MOdified Newtonian Dynamics.

If my reply implied that MOND was a relativistic theory then that was not intentional. From the first paragraph of the introduction to the paper I linked:

Our current best understanding of gravity is encapsulated by the theory of General Relativity (GR; Einstein 1915). This reduces to Newtonian dynamics in the weak-field non-relativistic limit...

This assumes that dark matter exists when there has been no direct evidence of it. The observations demand that the models change to fit the data. Some argue a light touch (dark matter).

Entirely on the contrary. Dark matter is the label given to a set of anomalous observations. The entire problem is that we called it “dark matter” when we should have called it “missing mass”, because that’s what it is. There would be no controversy with the latter name.

Technically, "missing gravity". The mass is inferred, but not absolutely required. (Though it is by far the simplest explanation.)

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