Outer stars in galaxies orbit the galactic center just as fast as stars closer to the center.

According to the known laws of gravity, stars on the outer edges of a galaxy should orbit slower than those near the heavy core. However, measurements of the Milky Way and other galaxies show outer stars moving at the exact same high speeds. This suggests a massive, invisible halo of dark matter surrounds the galaxy, providing the extra gravitational pull needed to keep these outer stars from flying off into deep space.