What is the Big Bang Theory? 

It was previously believed that the entire universe was the way it always had been “static”, and that it also only consisted of our milky way galaxy, until 1924 when a man named Edwin Hubble discovered that in fact the universe is much vaster then previously thought, with many more galaxies, and also that the universe is not static and is in fact expanding.This discovery along with Einstein's formulation of general relativity, lead to the theory that the universe must have had a point of origin in which it expanded from, thus the birth of the Big Bang Theory was conceived. The big bang theory states that the beginning of the universe began as a infinitesimal small volume with an infinitely high density and temperature, this was known as the “singularity”. This region of energy then began to inflate, expand and cool, dragging along with it the very fabric of space and time carrying all the observable matter we see today. It continues to expand today.

http://en.wikipedia.org/wiki/Big_Bang