What existed was the possibility of a big bang. The idea of "before" that is nonsense, as there can't be time without some sort of motion or change.
What existed after the bang wasn't nothing, it was something. In fact, it was the entire universe. But, because this something was so hot, it didn't contain any matter. It was all energy. When the energy cooled enough, it changed from energy into matter.
Kind of like making ice from steam - both are water - but not exactly the same as energy to matter doesn't preserve the chemistry.
And how is this a better explanating than God created it? Sounds like atheists go to a lot of trouble to deny God. Sounds just like most of the reasons to expand communism.
Pray for America