Big Bang Theory

Big Bang is a theory about the beginning of the universe. It is said that all the matter was packed close together, or, in other words that everything that we know today was present in a form of single body. Round about 15,000 to 20,000 million years ago, there was a huge explosion and all the matter was scattered in the space. That explosion is known as Big Bang. In the result of that big bang all the galaxies were formed and they are still moving away from each other  till now. Most of the astronomers agree with the theory.
Observation of galaxies proves that theses galaxies are still moving far from each other. And the universe is expanding with the passage of time.
The big bang consider the age of universe also. The explanations of big bang theory shows that at a time the universe was too hot and dense, therefore it began to expand rapidly. But after the initial expansion, it started to be cooled in such a way that the energy can be transformed into subatomic particles , such as electrons, neutrons etc. It is said that the first nuclei was formed within next few minutes after that great explosion that is known as Big Bang.
In short Big Bang theory is a scientific theory and is accepted by scientific communities of the world. There are other theories also about the universe such as, Steady State. But the Big Bang is most popular among all other theories.