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).
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