In the early 1970s, the astronomer Vera Rubin measured the rotation curves of spiral galaxies: the speed at which stars at different distances from the center orbit around it. According to Newtonian mechanics and general relativity, stars at the outer edges of a galaxy should move more slowly than those near the center, just as the outer planets of the solar system orbit the sun more slowly than the inner ones. They did not. Stars at the edges were moving far too fast. The visible mass of the ga...