[Beldin]

Actually, dark matter isn't the only game in town.

The whole need for dark matter arose from the fact that our theories (on gravity) didn't match some of our observations - while matching other observations phenomenally well.

Abstractly speaking, when theory doesn't match observations, either the theory is lacking, or the observations are. Dark matter is the hypothesis that the observations are lacking. The alternative is to come up with modifications of Einsteinian gravity that preserve accordance with observations where it's extremely good (e.g. perihelion shift of Mercury), and modify only the results for the parts where the match with observations is lacking. To the best of my knowledge, this category is collectively called MOND.

There are some good reasons to think that there exists a type of matter that we cannot observe - not that we didn't happen to see it like some small black hole in the middle of nowhere, but that our current instruments literally cannot see it, only its effects. There are also some reasons to think otherwise. The main one for that is that dark matter, by its nature, should be something. So not only should dark matter theories make predictions that can be proven, evidence for dark matter should eventually answer the question "what is dark matter?" That is a bit of a problem: right now, it might as well be unicorn farts.

[gpderetta]

IANAP, but I thought is that MOND is a modification of classical gravity and that GR-compatible extensions to MOND end up introducing dark-matter like terms.