Explanation:

Big Bang Theory (Expanding Universe Hypothesis)
It explains the origin of Universe. On witnessing that Galaxies move far from each other, Universal expansion is proved by Sir Edwin Hubble in 1920. For example if we take the balloon and mark some points on it, now blow the balloon we can clearly see the inflation leads to stretching of points.
Big bang theory can be explained in three developmental stages:
• At first tiny ball with small volume as an atom, which had infinite mass and temperature
• A violent explosion of tiny ball happened which resulted in huge expansion. change of some particles into energy form. Due to this expansion, some energy got converted to matter. This happened before 13.7 billion years from our present day.
• Then temperature dropped to 4,500K after 300,000 years of explosion, atomic matter is formed and universe become transparen