Big Bang theory was first proposed by Gammow and Ie maitre. According to them, millions of years ago, the whole universe was in the form of highly concentrated with compressed and dense matter acting as fire ball called primordial fire ball. Its temperature and density were so high that energy could not hold inside it and huge explosion of fire ball, Big Bang, took place so that the matter present in it started to scatter outwards resulting in the formation of universe.
The Big Bang is a theory that describes the beginnings of our universe. It contends that the universe began with a primordial explosion that occurred about 13 billion years ago. The original core of the explosion was very small and tremendously hot, composed entirely of high-energy radiation. As the universe expanded and cooled over the first three minutes, protons and neutrons were created out of that energy. All matter were compacted into a very small ball with infinite density and intense heat called a Singularity. Suddenly, the Singularity began expanding, and the universe as we know it began.