For a start: the speed at which stars rotate around the center of a galaxy. This should be due to gravity, yet the speed of stars further from the center isn't

For a start: the speed at which stars rotate around the center of a galaxy. This should be due to gravity, yet the speed of stars further from the center isn't slower (as it should be). This holds true for the vast majority of galaxies. A specific amount of extra (not yet observed) mass distributed in the right way throughout the galaxy could explain this.