Edwin Hubble discovered that the universe is expanding in all directions. If it's expending then it can be traced back to where it started. Traced far enough back and it all ends in an infinately small and infinately dense point. From there the Big Bang Theory explains how the expansion started. My point being that it's all based on evidence. In the 1960's the discovery of the Cosmic Background Radiation went a long way to proving the theory. The theory wasn't pulled out of thin air!