Nothing is...not for long anyway.Everything is in a state of flux and this implies that every object has a story behind it.However,the possible options for stories of origin are not satisfactory: 1.We can trace the origin of an object back to some arbitrary point in the past that we deem to be the 'beginning'-the Big Bang or God (depending on how religious you are). 2.We can come up with a chain of causality that never ends by questioning what happened before the aforementioned 'beginning'-What happened before the Big Bang...Where did God come from? 3.Finally,we can come up with a scenario where the origin of an object is incomprehensible,for example in backward time travel.Thought experiment:Suppose I took the complete works of Shakespeare and traveled back in time and gave them to him before he wrote them.He then duplicated the book I gave him and many centuries later,this duplicate became recognized as the works of Shakespeare-the book with which I traveled back in time to give to Shakespeare. None of these explanations seem satisfactory as explanations for the origins of objects. So,again,I ask:Where do things come from? P.S.I can't help but notice the similarity between this trilemma and Munchhausen's trilemma.
The expression "comes from" is ill-defined (in an extreme way - most terms we use in our daily languages are just "ill-defined" - because there's no obvious way to objectively define it). Where does smoke come from? Perhaps it comes from a burning object. Oh, stop it: it comes from a chemical reaction between oxygen and some other chemical component. Are you nuts!? Smoke is nothing but a chemical reaction, and all elements come, ultimately, from the Big Bang!
It all depends on where you would prefer to, mentally, stop that chain, beginning on the thing itself and ending with the Big Bang. There was no time before Big Bang, so "before" the Big Bang makes no sense: "before" is a time adverb, so we need to have time to have "before".
It seem's to me to make much more sense to simply understand and accept that chain, and try to build a valid model of it, something like a sequence of ordered (indexed) events. So when someone asks where does x comes from, you answer with the previous event in that chain. When you reach index 0 (or 1, for the MatLab users :p ), you simply answer: "there's no before when there's no time".
I propose that the question you ask may very well not apply when taken back before the Big Bang (or whatever even spawned the universe as we know it).
There is no information on what was around before that moment, and the... pliable nature of time shows that it is not some universal force existing outside of the physical existence. Indeed, both space and time (as we know them) may not have existed prior to that point. In that case, the beginning of all "things" could literally be the unbalancing of matter/antimatter that caused the Big Bang.
You really ought to read "-All You Zombies-" by Robert Heinlein.
But the short answer is that there is no answer. You've asked the ultimate question that science, religion, and philosophy have been attempting to solve for millenia.
My personal opinion, other than 42, is a closely guarded secret of Hindus, Jews, and a few Christians who manage to study Exodus in Hebrew. And Descartes. The noun "I AM" is a very interesting verb, in Hebrew.
Empty space is most likely not empty. It has some structure on the Plank scale which some have referred to as quantum foam. Theorists have long agreed that singularities don't actually exist. The likely don't exist because a single point of infinite density isn't possible. Therefore, the singularity that was just sitting there waiting to explode in a "Big Bang" likely didn't exist either.
Our universe is expanding, it has a border. It's expanding into something. That something is likely a multiverse.
By all of this, we're beginning to suspect that black holes aren't focusing all of the matter it swallows into a ultra-dense point.
One idea is that our universe came into being from a "white hole", i.e. the end of the digestive tract of a black hole.
Just a thought...
I'll address just a specific part of your question:
What happened before the Big Bang...Where did God come from?
That question is based on the hypothesis that there is something before the Big Bang. And that might not be true, despite not being too intuitive (as many phenomena of nature). You can ask that question only if you know for sure that that hypothesis is valid, and today we do not have that sureness.
Some cosmologists/physicists think that that question makes no sense because they believe that the mentioned hypothesis is false. Therefore, in this case, the answer is simply:
There is nothing before Big Bang. Space and Time started there; before, there's nothing but the 'true' nothing.