It was Albert Einsten that created the theory that gravity is the dominant force that controls the universe.
Cosmology is the study of the universe as a whole.
The Big bang theory is that the universe all started from small point no bigger than a coin. That little point exploded starting off the world as we know it today. The theory behind it is that the bang started off at one spot but later it happened at other spots simutaneously. We do know that the universe is still expanding. And that it expands with explosions just like how we all started off from that small little point surrounded by nothing.