[NZGumboot]

You're right that there could be multiple things which separately explain the things that dark matter explains (not just the two you listed, but the others as well: gravitational lensing, galaxy cluster motion, etc). But the strength of the dark matter model is that it can explain all of these phenomena; which is why most physicists prefer dark matter over other theories. It's very easy to come up with a theory which explains a single phenomena, but very hard to come up with one that simultaneously explains multiple phenomena.

[dnautics]

To play devil's advocate, the strength of a model alone is not sufficient: I could say: Well, how can we be certain "gravitational waves" detected by LIGO aren't random dark matter density ripples and not GW from events we haven't correlated (n=1 on independent correlations). In other words, the dark matter model is too strong, because there is a arbitrarily parametrizable density field... Hell, there are people postulating Sagittarius A* is a very dense ball of DM and not a black hole. At that rate you can explain anything with dark matter. I have no logical or philosophical basis for this but in general when this sort of thing starts happening I start doubting the validity of a concept, not gaining confidence in it.