Here’s the thing: the outer regions of galaxies in our universe rotate at the same speed closer to the center, which contradicts Newtonian mechanics in which the rotational speed should decrease with distance from the center due to lesser gravitational pull.
However, that doesn’t happen, and everything rotates at the same speed, suggesting that there’s additional, unseen mass that provides the necessary gravitational pull that maintains speed. Thus, scientists have come up with the concept of dark matter, and they’re now trying to confirm using of black holes.