If true, there was not just one Big Bang, there were many Big Bang. Each Big Bang made each galaxy, as each galaxy has its own Black-hole. The size of the Black-hole now, was probably the size of the dot, before the Big Bang. So, it was a very large dot, and not a very small dot.
How did these Big-Bangs Happen? I think two Black-holes found each other and attracked each other, since they have great gravity forces. They crashed into each other so hard, that it exploaded there matter all over the place.
It has been said on the History Channel, that the Universe is Expanding. Correction. The Universe is not expanding. Only part of the Galaxies in the Universe is exspanding in the Universe, but not the Universe it self., while the center part is being eatten by the Black-hole. The Universe can't exspand, because I believe that the Universe is not measurable at this time. The Deep Hubble Telescope has only seen up to 47 billion light years, into the cosmos, but we are limited to see beyond that, due to todays technologies. If you can find a wall that say, "The End". Let me know.
In order for the Big-Bangs to happen, it has to have a Universe to take place in. In other words, these Big-Bangs did not make or create the Universe, but only the displacement of the junk and stuff that is with-in Universe.
You also seem to be implying that we know something about the actual "Bang" of the big bang...we do not. There is a lot of scientific speculation but we just don't know enough about the nature of the universe to begin to make even an educated guess.
There is an "edge" to the universe. As you look farther out into space, objects are found to be traveling ever faster. In fact, much to our current astonishment, it seems to be ACCELERATING! This behavior implies strongly that there is an "event horizon" associated with the universe as we can never observe any massive body traveling at the speed of light.
As for your last assertion...why does there have to be a universe for it to happen in? Again, I do not believe we are even close to understanding enough about the universe we recently woke up in to make such an assertion.