Question

What existed before the Big Bang and what started it?

Answer 1

There are several hypotheses about the origins and/or cyclic nature of the universe, but none has taken preeminence. Some options are Nothing, Final Contraction of the Previous Universe, and Chaos.

Explanation:

Author Terry Pratchett (RIP) made some interesting speculations. Dr. Stephen Hawking has one of the most balanced reviews of current theories and possibilities. Check out his very readable books on the History of Time.
It may, or may not, have had a "start".