Question

If the Universe was born at the Big Bang, what existed before then?

Answer 1

The scientific explanation known as the Big Bang posits that the explosion started with a hypothetical singularity.
This initial singularity was of infinite density, sort of the mother of all black holes. The density reached a point that the heat it produced caused quantum fluctuations, an explosion, such that it formed the entire universe as we know it.
The created universe would have replaced what ever was there before.
Some theorize that this is a process that occurs over and over again. So there was a universe, it contracted and exploded forming what we have now. This view would further posit the the universe we now know will eventually go through the same process.