dark energy definition of dark energy in the Free Online Encyclopedia.

dark energy, repulsive force that opposes the self-attraction of matter (see gravitation gravitation, the attractive force existing between any two particles of matter .

The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it
..... Click the link for more information. ) 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 Hubble Space Telescope (HST), the first large optical orbiting observatory . Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble ) was expected to provide the clearest view yet obtained of the universe.
..... Click the link for more information. of a distant supernova that implied an accelerating, expanding universe, which in turn required a new cosmological model (see cosmology cosmology, area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe .

Modern Cosmological Theories

..... Click the link for more information. ). 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 c.70% dark energy, c.30% dark matter dark matter, material that is believed to make up (along with dark energy ) more than 90% of the mass of the universe but is not readily visible because it neither emits nor reflects electromagnetic radiation , such as light or radio signals.
..... Click the link for more information. , and c.0.5% bright 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.

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.