"what is the bigbang universe expanding into ............ itself? "
That's a good question. We don't really know. After all we are mere babes as far as knowing how all this works.
One thing is certain - the Universe is expanding. Anyone who doesn't believe in this is not only illogical but trying to spread lies in the face of fact – and it’s cooling, which is another fact.
So, what is the Universe expanding into? Well it’s not! I know this sounds like a contradiction. Let me explain.
Regardless of which theory of the shape of the Universe you take (open, flat, closed), you can never reach the edge of the Universe as we know it. It would be like trying to go further North than the North pole. Another analogy is back to the hypothetical 2d balloon world. In 2d on the surface of the balloon there’s no concept of up or down so you can never reach the edge of the balloon, and you can’t see what the balloon is expanding into. Once you introduce the 3rd dimension you can see what it’s expanding into. Same idea with our Universe. Latest string theory suggest there are other Universes interwoven with ours all in hyperdimensional space, and this is what our and the other Universes are expanding into. I take such theories with a grain of salt. We’ll probably come up with better explanations one day but for now that’s the most common explanation.
There’s one possible flaw in all this. How can a 2d balloon exist at the same time and space as the 3d version we can all see in our universe? Would we be able to see the 2d creatures crawling on their flat 2d Universe on what we would see as a 3d sphere? String theory? E8 theory?
