Yes, virtual particles fluctuation theory. The problem is we have no evidence of it before the big bang. Then, there is the argument based on time, where did that vacuum with VPs come from? At what point in time, or from where did it derive. Assuming physical laws, we must have an explanation for origins.
I know about God of the gaps. This however this is a huge gap. Also, I am not proposing that science is wrong in most explanations, I'm assuming God caused the big bang and science explains what happened thereafter.
It is interesting that science has not seriously regarded laws of probability for cosmological events. As an example, what are the probabilities for matter and anti-matter producing galaxies? what are probabilities for life happening on earth, one of billions of planets?
On the question of nothingness, that mainly depends on how you view the vacuum:
(1) The universe (something) came out of the vacuum (nothing).
(2) The universe (something) came out of the vacuum (something).
If you take position (2), then to the question, how did something came out of nothing, the right answer is, it never happened.
Now if we take into account that there are other scientific theories floating around: Reinhartd's cyclic theory, Susskind's multiverse, Smolin's fecund universe or Penrose' conformal cyclic cosmology theory. We need more data to filter some of these. But science is not short on this topic.
As to the question of time: according to GR, spacetime is a dynamical feature of the universe. IOW, universe and spacetime can only exist together.