The question of why something and not nothing is not proof anything really. The fact that "something" was in existence in the beginning does not *mean* anything.
When people think of "nothing" they tend to picture an immense blackness. Like space without the starts. Unfortunately, in a realm of "nothing" there wouldn't wouldn't *be* empty space. There has to be "something" for something to even be empty. That's why space (the literal space, not what we call the expanse outside of earth) didn't exist until the beginning.
This is the problem with trying to explain the "big bang" to people who don't understand. They usually see the "egg" as sitting in the middle of "nothing" and then exploding outwards and "filling" "nothing" up with "something". This is mostly, (unfortunately) because it is the best analogy we can come up with right now. But in reality, the egg *wasn't* sitting in the middle of "nothing". The *only* "something" in existence was the egg. And then it... hatched I guess you could say (lol) which is another bad analogy because it assumes something laid the egg... You see? By attempting to explain I can only go in circles. The point is, "something" didn't *have* to come from "nothing", for just as "nothing" could always exist, (and indeed can be said to still exist, side by side as before, but that's another conversation...) "something" would also simply always exist. It's in fact *necessary* for "something" to have always existed. For even if you are a deist of some type, any God would be "something" even if not a physical "something". The only argument between theists and atheists is whether the original "something" was the matter we already have proof of existence, or the spiritual matter that science has no currently accepted proof of. The two ("nothing" and "something") have equal values on the scale. It's a multiple choice question with only two answers. Take your bet. Of course there's the hidden third answer, where they both exist, "something" being the universe, and "nothing" being outside it and yet also not in some other universe... But back to the point, if matter and energy can never be created or destroyed, only changed from one form into another, even on the extreme going from matter to energy and vice verse, then by *definition* we already understand that it always was, and had no beginning.
As for the physics, trying to understand physics from the analogies is really a bad idea. You can't do it. The analogy is so broken down, and leaves out so many tiny (and not so tiny) components in an attempt to bring the student even a tiniest bit *close* to understanding, that attempting to prove or disprove anything with the analogies is not possible unless both people actually understand the real physics behind them and so can understand the math that is needed for the analogies. For anyone else, the analogies leave so much out that you can't argue anything with it, simply because you have no idea what you're actually arguing.