(cosmology) the theory that the universe originated 20 billion years ago from the cataclysmic explosion of a small mass of matter at extremely high density and temperature
In astronomy the big bang theory is a theory that suggests that the universe was created as a result of an extremely large explosion. A cosmological theory holding that the universe originated approximately 20 billion years ago from the violent explosion of a very small agglomeration of matter of extremely high density and temperature. the idea that the universe began with a single large explosion (the 'big bang'), and that the pieces are still flying apart steady state theory