What is the universe expanding in to and how far can it expand?

If I'm quite honest no one really knows the answer. However, there are a lot of interesting ways to look at it. My favourite way is to imagine that the world was two dimensional, so you and the Earth are stuck on a flat bit of paper. Now imagine that that piece of paper is part of a giant globe (you can also think of it as a flat spot of ink on a balloon). As the balloon expands, it's expanding in three dimensions, but the surface area you can see as a two dimensional human being will be getting bigger and bigger and bigger. Now if you can imagine that we see the world in 3D, if you can imagine that was maybe just the surface, as it were, of a four dimensional shape, (I really struggle to imagine it) then that would be one way that you could imagine it. There are more dimensions that we can actually see and we are expanding in these other dimensions. A lot of people ask what is it expanding into, because there must be something. But of course if the universe is everything, then as soon as it expands it just makes the thing that exists bigger. There are also ideas that there are millions of different universes all competing for space and going at different speeds. Like little bubbles, all crowding each other out. And actually, maybe the black holes in our universe go through to other universes that are being created. It's really one of those questions that's more philosophy almost, than science. Everyone's really only making guesses. Everyone's trying to get the evidence to prove that it's one way or another and that evidence just isn't here yet.