Nearly 100 years ago Edwin Hubble discovered that the universe is expanding: almost all galaxies are speeding away from our own Milky Way, and faraway galaxies are receding faster. That discovery was profound, but it was followed, in 1998, by an even more startling realization: the expansion is accelerating. For most of the 20th century, scientists had expected that over time gravity would pull galaxies toward one another, slowing the expansion. Instead two teams of astronomers studying supernovae—exploding stars that serve as markers for measuring cosmic distances—found that the growth of the universe is actually speeding up. This remarkable discovery, since confirmed by other observations, was awarded the Nobel Prize in Physics in 2011. But why is the universe accelerating? This enigma is one of the biggest unsolved mysteries in all of science.