dark energy, repulsive force that opposes the self-attraction of matter (see gravitation) and causes the expansion of the universe to accelerate. The search for dark energy was triggered by the discovery (1998) in images from the Hubble Space Telescope of a distant supernova that implied an accelerating, expanding universe, which in turn required a new cosmological model (see cosmology). Although dark energy is predicted in particle physics, it has never been directly observed. It is generally agreed, however, that dark energy dominates the universe, which is projected to have a composition of about 68% dark energy, 27% dark matter, and 5% normal visible matter such as stars. By 2006, astronomers using the space telescope to examine more distant supernovas had found evidence of the effects of dark energy dating to 9 billion years ago. Observations with the Chandra X-ray Observatory, published in 2008, indicate that dark energy is also retarding the growth of distant galaxy clusters.

The concept of dark energy was first proposed, and then discarded, by Albert Einstein early in the 20th cent. His theory of general relativity implied that the pull of gravity would make the universe collapse, but, like many scientists of his time, he assumed the universe to be static and unchanging. To make his equations fit these assumptions, Einstein added a "cosmological constant" whose effect was repulsive. When American astronomer Edwin Hubble discovered that the universe was expanding, it was assumed that the universe must be slowing down because of gravity and might even come to a halt. This led Einstein to remove the cosmological constant from his equations and to say that it had been the biggest blunder of his career.