View Larger Version | Early in the universe, the cosmos was compact and the density of dark matter so high that it slowed down the expansion that began with the Big Bang. At such early times, by comparison, dark energy's push was almost inconsequential. But as the universe grew bigger, and the density of dark matter diminished, dark energy's outward push grew stronger than dark matter's inward pull. Dark energy began to dominate about 5 billion years ago. Ever since, the universe has expanded at an accelerating rate. Credit: NASA, adapted by J. Korenblat; Galaxies image: NASA, ESA, J.Blakeslee/Johns Hopkins, M. Postman/STSCI, P. Rosati/ESO