What was the Big Bang?

The Big Bang is the most popular theory about the creation of the Universe. According to it, the whole Universe was created in a split second in one huge explosion. All matter was squeezed together into a tiny, super-hot, dense ball that was smaller than an atom. The ball gradually expanded as it cooled, then exploded, releasing energy and matter in all directions. We cannot see the Big Bang because it would have happened billions of years ago. But we can see that the Universe is growing steadily bigger. All the galaxies are speeding away from each other as the Universe expands.