Flexi Says: The Big Bang theory is the most widely accepted cosmological explanation of how the universe formed. According to the Big Bang theory, the universe began about 13.8 billion years ago. Everything that is now in the universe was squeezed into a very small volume. Imagine all of the known universe in a single, hot, chaotic mass. An enormous explosion — a big bang — caused the universe to start expanding rapidly. All the matter and energy in the universe, and even space itself, came out of this explosion.