What Holds the Universe Together

In the 1970s, astronomer Vera Rubin pointed her telescope at the Andromeda galaxy and discovered something that should have been impossible. The stars at the galaxy's edge were spinning far too fast. By every known law of physics, they should have flown off into the void. Something unseen was holding them in place. Rubin had stumbled onto dark matter — an invisible substance that makes up roughly twenty-seven percent of the universe.…