Dark matter
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In the 1980s astronomers discovered that the visible matter in rotating galaxies has insufficient gravity to hold the galaxy together. The term dark matter refers to whatever is generating the extra gravity.
Academic physicists feel that most of the mass in a galaxy is unseen dark matter, and that known substances like [neutrino]s, black holes, and dead stars are insufficient. Many physicists believe that the dark matter is some as-yet-undiscovered subatomic particle, such as the axion, Higgs, or lightest supersymmetric particle.