Two teams of cosmologists simultaneously stumbled upon the second dark continent in the 1990s. The movement of distant supernovas revealed that the expansion of the universe was picking up speed. Some sort of "dark energy" was working against gravity to drive galaxies apart faster and faster. Theorists immediately identified the prime suspect: a tiny amount of energy intrinsic to the vacuum of space. Albert Einstein had long ago considered this possibility, dubbing it a "cosmological constant." Such energy would have had a negligible effect early on when the universe was small, but its effect ?" repulsion ?" would grow with the size of the universe. In the very long run, the increasingly rapid expansion is expected to dilute visible and dark matter to imperceptible levels, leaving an explosively expanding universe made up almost exclusively of dark energy.