Cosmic expansion is speeding up. In 1998, astronomers showed evidence from exploding stars that the expansion of the universe is not slowing down because of the gravity of matter in the universe. By observing the light from distant supernovae, astronomers can measure the distance to each explosion. By studying the redshift in the supernova's spectrum, they can measure the expansion of the universe since the light was emitted and, from a set of these measurements, can trace the history of cosmic expansion. Instead of seeing a slowing universe, as expected, observations indicate there is a component of the universe, called the dark energy, that makes cosmic expansion speed up. Dark energy comprises about two-thirds of the energy density of the universe. While it may be related to Albert Einstein's controversial cosmological constant, its true nature remains to be discovered by observation and thought.