As you may already know, at the very beginning of the Big Bang all the matter in the universe was concentrated at a single point (a
singularity). There was a tremendous explosion (that's the "bang") and this matter began expanding, and it is still expanding today. Now, asking what happened before the Big Bang occurred is a little tricky. There is no way for us to know, just like there is no way to know what's inside a black hole. Physics as we know it breaks down in a singularity, be it inside a black hole, or at the Big Bang.
Since scientists are unable to know what happened before the Big Bang, we often regard it as the beginning of the universe, and the beginning of time itself. (See, that's a sneaky way to avoid the question, since if the Big
Bang is the beginning of time, then there was no
"before" the Big Bang!)
Your question is a very good one. And it's something we'll probably never know. But it could be (this is a theory, we don't know) that the universe won't keep expanding forever. Maybe eventually it will stop expanding,and begin to fall back together. This theory is called the Big Crunch.
In this theory, eventually the universe would come all back together to a point. Then there could be another Big Bang, the universe would be "reborn", expand again, before the next Big Crunch. And so on.
So it could be that right before the Big
Bang, there was a Big Crunch, but this is still
being debated by scientists.
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