What was the Big Bang Theory?

The Big Bang is the most popular theory about the creation of the universe. According to this theory, the whole universe was created in a split second by one huge explosion, which is why it was called the "Big Bang". According to it, 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 haven't witnessed the big explosion because it would have happened billions of years ago. However, it was noticed that the universe is growing steadily bigger. It was observed that the galaxies are speeding away from each other, as the universe expands.