For the first ten billion years of the existence of the universe the attractive forces of gravity on matter—both ordinary and dark—slowed the expansion of the universe. But beginning about five billion years ago the universe began to expand at an increasing rate. Two studies of supernovae have documented this acceleration. The cause of this expansion has been called dark energy, but the nature of dark energy is totally unknown. It interacts only via gravity and is very dilute. One possibility is that Einstein’s General Relativity has to be modified by the addition of a cosmological constant. Einstein himself considered such a constant, then discarded it, calling it his biggest blunder. The problem with such a constant is that particle physics estimates the value (in proper units) of 1, but the value required to explain the extra expansion is 10-120. This huge discrepancy cannot be explained.