Dark matter is more of an observation than a theory. The gravitational motions of galaxies doesn't really make sense without it (and with dark matter + General

Dark matter is more of an observation than a theory. The gravitational motions of galaxies doesn't really make sense without it (and with dark matter + General Relativity, it fits perfectly, so we have good reason to believe it exists). There are a few different theories on what dark matter is; there's axions, supersymmetry (from string theory), cold dark matter and more.