Big Bang. Conceptual computer artwork representing the origin of the universe. The term Big Bang describes the initial expansion of all the matter in the universe 12-15 billion years ago. The primordial universe was in an infinitely compact state, much smaller in size than an atom. Less than a second after the Big Bang, temperatures reached trillions of degrees Celsius, and the universe has been expanding and cooling ever since. Matter formed and coalesced into galaxies, observed to be still moving away from each other today as they are carried by the expanding universe. Background radiation in the universe is considered a remnant of the Big Bang.