I guess what is different about dark matter is that it has to outmass regular matter by a large factor. It feels unparsimonious to invent four-five times the mass of the known universe just to patch a discrepancy between observations and a theory of gravitation. It feels like the theory would better be adjusted to match observation than to patch observations to match theory.

Today I learned that the mass of the neutrinos we know about (which were similarly invented, though since detected) about matches the mass of all the stars.