Whole cosmic system of matter and energy of which Earth is a part. Its main constituents are the galaxies, within which are stars and stellar groupings and nebulae (see nebula). Earth's Sun is one star among the billions of stars in the Milky Way Galaxy. All atoms, subatomic particles, and everything they compose are also part of the universe. The universe is governed by four fundamental forces: the strong force, the weak force, the electromagnetic force, and gravitation. Numerous theories have been proposed for the origin and structure of the universe. See also big bang; cosmology; expanding universe; steady-state theory.
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