n.
Whole cosmic system of matter and energy of which Earth is a part.
Its main constituents are the galaxies , within which are star s 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 .