THE BIG BANG
The big bang theory is the idea that the universe had a beginning. According to the Big Bang Theory, the universe began with a single explosion of nearly infinite density, and infinite energy. Proof of this theory includes the expanding universe, background radiation and the composition of matter. This singularity would have produced an immeasurable amount of energy. Albert Einstein's theory of relativity started it all, from there we have Hubble's expanding universe and Hawking's proof of beginning. Some theories opposing the big bang are the Steady State theory and String theory. Steady State is the theory that the Universe has always existed, and that it has always been expanding at a steady rate, with hydrogen being the abundant element. String theory is a theory connecting the universe and all matter through a system of cosmological strings.
Evidence: