The origin of universe is best classified by the Big bang theory. This theory proposes that the universe is very old and its origin took place almost 20 billion years ago. It states that a single huge explosion i.e. Big bang took place in the space which was so powerful that it is unimaginable in physical terms. Due to this explosion, the universe expanded in volume and the temperature of the space came down. Slowly hydrogen and helium gases formed, which condensed due to the gravitational forces present in the surroundings. This all resulted in the formation of billions of galaxies.