When Albert Einstein was working on his equations for the theory of general relativity, he threw in a cosmological constant to bring the universe into harmonious equilibrium. But subsequent observations by Edwin Hubble proved that the universe was not static. Rather, galaxies were flying apart at varying speeds. Einstein abandoned the concept, calling it the biggest blunder of his life’s work. Observations in the 1990s, however, proved that the universe was not only flying apart, it was doing so faster and faster. This seemed to point to a dark energy filling space that actually repelled ordinary matter with its gravity, in contrast to all other known stuff, including dark matter. A number of theories have been developed to explain what this dark energy might be, including Einstein’s long discarded cosmological constant.