The accelerated expansion of the universe is the observation that the expansion of the universe is accelerating, as opposed to decelerating. This accelerated expansion was first discovered in 1998 by two independent teams of astronomers, who found that distant supernovae were dimmer than expected. This observation suggested that the universe was expanding at an increasing rate, rather than at a decreasing rate as had been previously assumed. The cause of this accelerated expansion is still unknown, but it is thought to be related to a mysterious form of energy called dark energy.