Big Bang, conceptual illustration. An artwork representing the origin of the universe until the Universe today with our Earth and Moon and Sun in our Milky Way.. The term Big Bang describes the initial expansion of all matter in the universe from an infinitely compact state 13.7 billion years ago. The initial conditions are not known, but less than a second after the beginning temperatures were trillions of degrees Celsius and the primordial universe was smaller than an atom. The universe has been expanding and cooling ever since. Matter formed and coalesced into galaxies, which are observed moving away from each other. Background radiation in the universe is considered a remnant of the Big Bang.