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Nonluminous matter not directly detectable by astronomers, hypothesized to exist because the mass of the visible matter in the universe cannot account for observed gravitational effects. Dark matter comes in two varieties: baryonic, which is about 5 percent of the universe, and nonbaryonic, which is 22 percent of the universe. The nonbaryonic dark matter is believed to consist of heavy, electromagnetically neutral particles called weakly interacting massive particles (WIMPs).
The dominant component (about 70 percent) of the universe. Its nature is not well understood, but it differs from matter in being gravitationally repulsive. Dark energy is detected by its effect on the rate at which the universe expands. Many explanations have been proposed for dark energy; the simplest is that it is an energy density inherent to empty space.
© 2010 Encyclopædia Britannica, Inc.


