I can't talk much about the specific theories that serious scientists talk about, because it's based on math that goes far beyond what I've ever managed to wrap my brain around. (I was considered a math genius in high school; I understood and was fully comfortable with basic calculus. But my chosen educational path stopped before I reached vector analysis, real analysis, or more than baby steps into differential equations, and when I've made tentative overtures like trying to look on Wikipedia to figure out what a tensor is, I've just found myself drowning in unfamiliar notation.) So some people would probably argue that I have no basis to have opinions, but being the arrogant snot that I am, I still have them.
One of the opinions that I have is that, while there are tens thousands of people out there who have PhDs in theoretical physics (or closely related fields) and can confidently sling around equations, pretty much none of them have any real intuitive understanding of what any of the math means. Physicists say that if you can't explain it with equations you don't really understand it, and I do understand and agree with that, but I contend that the converse (or obverse or whatever the proper term would be) is equally true: if you can't truly explain it with just words and concepts, without resorting to equations to justify the reasoning, you also don't understand it. And without that full understanding, it's highly unlikely that the explanation is really right.
The basic understanding of Big Bang cosmology, at least as I understand it, doesn't forbid the idea that there are other universes with other bangs, but it postulates that we can never actually experience them. I've heard some cosmologists wandering on the border with mysticism claim that the question of what happened before the big bang is not merely beyond what we can learn experimentally, it's fundamentally meaningless. Personally, I think that their concept of time as a dimension mathematically equivalent to the spatial dimensions is, to put it bluntly, bogus. If you assume that time is a dimension you can conceive of space-time as a single object, and for some purposes it's a useful and compelling conception, but if time is just one of the directions that the universe blob extends in, how do you explain the notion of "now"? Yes, you can describe "now" as a slice through the four-dimensional blob of space-time, but what constrains our perception to that slice, and more importantly, how can that slice "move" through the blob to create what we perceive as the passage of time? We defined time away at the start of the exercise, and motion is change in position with time; to explain our universe without time as we know it, we have to invent time, which falsifies our initial assumption.