The Big Bang is a theory that astronomers believe 13.7 billion years ago, a giant explosion created the universe in a fraction of a second. The universe expanded rapid from the size of an atom to a hot fireball with size bigger than galaxy in that Big Bang moment. The universe is stil expanding although some scientists believe that the gravity of dark matter has slowen down the expansion of the universe. For more, you can watch the videos about the Big Bang on BBC website.