The Big Bang is a scientific theory that postulated how the universe came into existence. Matter, energy, space and time are all have been thought created in a fraction of second 15 billion years ago, when there was colossal explosion called the Big Bang that created tremendous heat. Nobody knows what caused the Big bang, since one cannot look back to a time before it. But just at the time of the Big Bang, the universe was less than the size of a pea, and its temperature was ten thousand trillion, trillion degrees Celesius.

After the Big Bang, the universe started expanding and cooling. First hydrogen and helium were created within the first fifteen minutes. When these clouds of hydrogen and helium are collasped under gravity, stars and galaxies are formed. Later planets began to form. It is estimated that there are about 100 billion galaxies in the universe, and each galaxy containing 100 billion stars.