The Big Bang Theory

In 1929, Edwin Hubble and Milton Heimerson made empirical observations that showed without a doubt that our universe is indeed expanding. The most mind-boggling thing about this to me is that the universe is not actually expanding into anything else at all. So for the universe to look like what it does now, it must have in the distant past been in a much more dense and hot state so that it could expand out and cool to what we see today. This is the basic idea behind the Big Bang Theory.