Einstein predicted the existence of Dark Energy but later called it the greatest mistake of his life.
New evidence from the observation of a supernova by the Hubble Space Telescope, whose light is just now reaching us after having exploded 11 billion years ago, suggests that Einstein was right once again. In short: the light from the supernova is dimmer than would be expected and suggests that the universe has recently increased its rate of expansion. Here is an excellent analogy from a CNN.com article: "The new stellar explosion has helped astronomers understand how the universe expands, 'much the same way a parent follows a child's growth spurts by marking a doorway,' said Hubble scientist Adam Riess, lead researcher in the new study." The universe's "growth spurts" could be explained by Einstein's Dark Energy theory.