My theory is that another universe existed before this one, with different physical constants and fundamentals, at a higher-energy vacuum state. That universe decayed to a lower-energy vacuum state in a process known as a vacuum metastability event, which redefined these constants and fundamentals and gave rise to the universe as we know it.
A vacuum metastability event starts out as a point, and is spontaneous. It expands at very nearly the speed of light (if not at that speed), and inside the expanding bubble, I imagine the freshly-redefined constants and fundamentals would be churning chaotically and violently in much the same way our universe was in its earliest moments.
If this theory turns out to be true, it could also be true that some form of intelligent life existed in that prior universe, although I doubt we could ever know for sure even if we attained some kind of god-like understanding of our universe.
There is also the possibility that our vacuum energy state is another false one, and that our universe could decay to another lower-energy state at some point in the future. We are actually rather close to determining this with some level of certainty; an answer may be mere decades or even years away at most, depending on what we find out in studying the new Higgs Boson particle that was recently discovered and confirmed.
"We don't know." Is the wholly appropriate and accurate answer to this question. Although we can use what we know about the universe, and use this to rewind the clock back to the moments before the Big Bang, we haven't yet got back to what was or happened before. It may not even be something we can detect or discover- although I wouldn't place my bets on that. So, for now, we can only really use some ideas we know are true about the universe, to give some based assertions, postulations and hypotheses on what was before.
I'll start with saying that the old model of the big bang theory is severly flawed.
Due to two main problems,
The flatness problem
The horizon problem.
We'll jump into these two issues after a brief introduction to the theory of "Chaotic Inflation" .
Now the big bang speaks of an extremely dense material exploding, and the issue is where did that come from? Why where and why is space itself expanding faster than the speed of light and it is doing that exponentially.
So let's imagine, nothing. You can just say pitch black, this is as far as our minds can go at the moment so lets just do that.
Now we have vacuum, which is nothing. ( I am speaking of the 3rd dimension or the spatial and not other dimensions. )
This vacuum will inflate creating space.
And as it does this, quantum fluctuations starts where particles exist for sometime in our 3rd or spatial dimension. And since at the beginning there is no energy in the universe they exist longer obeying E=MC2.
But now there is a mathematical issue, we can't say 0 brings about +1 for example, 0=1 is invalid!
But we know gravity is negative and only equal to 0 at point infinity which does not really exist, therefore the +1 value created has an opposite -1 value created due to gravity.
So 1-1=0, 0=0 is totally valid!
Even every positive charge created will have it's negative charge immediately and simultaneously created.
So we can say, vaguely the universe is itself equal to 0, which means it just a manupilation of 0.
So the universe does not need energy to be created! Since it's sums are in the end =0 . But that doesn't explain the precise laws of the universe and the 4 forces. Which is really another issue..
Now the issue of the flatness problem is how come if the universe has exploded and is expanding, spacetime has not expanded?
Spacetime or the 4th dimension is extremely flat and we have somewhat probed it and found it is such and obeys the predictions of Einstien.
So, we said both sums are = 0, so really it won't expand since the resultant force is equal to 0.
And I do not have much information on the horizon issue so I will not indulge into a topic I do not have much detail about.
Hope this clears it out, tried to keep it simple!
And you might want to search for the theory of "Chaotic Inflation" on a search engine.
Stephen Hawking says that what happened before the big bang is irrelevant, because all information before it is inaccessible, and that we may as well consider the big bang as the beginning. It is possible that the big bang occurred from the collapse of a previous universe, and is now bouncing back out; for example. However, we may as well conclude that our big bang really is the genesis - because it would just be a question of "what came before the universe before us?" and we could eventually have to explain how a universe came from nothing at all...
i suggest "A Universe From Nothing" a great book by Lawrence Krauss