Explain the Big Bang theory in the context of the origin of the universe.

(N/A) The universe is approximately $20$ billion years old. It contains huge clusters of galaxies. Galaxies contain stars and clouds of gas and dust. Considering the size of the universe,the earth is indeed a speck.
The Big Bang theory attempts to explain to us the origin of the universe. It suggests that a singular huge explosion occurred,which is unimaginable in physical terms.
The universe expanded and hence,the temperatures came down. Hydrogen and helium formed sometime later. The gases condensed under gravitation and formed the galaxies of the present-day universe. In the solar system of the Milky Way galaxy,earth was supposed to have been formed $4.5$ billion years back.