The Universe started with the Big Bang about 14,000 million years ago (14,000,000,000 years). It started out very, very hot and has been expanding and cooling since then.

For the first tiny fraction of a second, everything was so hot, that we cannot describe what it was like, but we can use our knowledge of physics to give us a good idea of what was happening from as little as 10^-43 seconds (that is 0.0000000000000000000000000000000000000000001 seconds!) after the beginning.

Astronomers think that the Universe started with the Big Bang. As with all science, this is based on evidence; so what is the evidence for the Big Bang theory?