> there must be something else No, it could also turn out that Einstein's field equations require modification at certain scales or in certain situations. "Da

> there must be something else No, it could also turn out that Einstein's field equations require modification at certain scales or in certain situations. "Dark matter" is just a term to describe hypothetical matter that would cause the observed deviations from General Relativity – if it were actually there. People have proposed other ways to explain those deviations, though, like MOND, f(R) gravity, entropic gravity and many others, see https://en.wikipedia.org/wiki/Dark_matter#Alternative_hypotheses https://en.wikipedia.org/wiki/Dark_matter#Alternative_hypoth... .