Because it's the most economical assumption. We know that there are various different particles and there are some that we haven't discovered yet. There's no r

Because it's the most economical assumption. We know that there are various different particles and there are some that we haven't discovered yet. There's no reason why some particle wouldn't only interact via gravity and weak interaction. Adding some "dark" mass in the simulations paints the picture that matches what we see. Some galaxies were found without these gravitational anomalies, which all but rules out a general systemic issues with the theory of gravity.