We Never Know
No Slack
And Science without a doubt does not have experience of stuffs popping into being ex nihilo sine causa. Bohmian mechanics, for instance, is completely deterministic and furthermore emphasizes that every indeterminacy is actually conceptual.
“Being never arises from nonbeing,” “something will not originate from nothing” are putative metaphysical principles, just like cause and causatum, unhindered in their application. Hence, we certainly have excellent grounds, both abstractly as well as scientifically, for reasoning that whatsoever begins to exist, as in the case of the universe 13.7 billion years ago, has a cause.
What do you think/believe existed before the universe did and where did it exist?