What does the Big Bang theory describe?

Galaxies contain stars and clouds of gas and dust. Considering the size of universe, earth is indeed a speck. The Big Bang theory attempts to explain to us the origin of universe. It talks of a singular huge explosion unimaginable in physical terms. The universe expanded and hence, the temperature 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 about 4.5 billion years back. There was no atmosphere on early earth. Water vapour, methane, carbon dioxide and ammonia released from molten mass covered the surface. The UV rays from the sun brokeup water into Hydrogen and Oxygen and the lighter H₂ escaped. Oxygen combined with ammonia and methane to form water, CO₂ and others. The ozone layer was formed. As it cooled, the water vapor fell as rain, to fill all the depressions and formoceans. Life appeared 500 million years after the formation of earth, i.e., almost four billion years back.