What is the universe expanding into? Why need not something be there?!
The universe need not be expanding into anything at all. Taking the 2D analogy: a 2D spherical balloon is, in our human experience, expanding into 3D space. But suppose your balloon universe is all that exists. If you are an ant on the balloon, you can observe your universe expanding, but have no concept of 3D space. There is no mathematical need for 3D space to exist: the whole universe is just a 2D balloon, getting bigger; the third dimension can be “unphysical”. Similarly, we are 3D beings observing that our 3D space is expanding. We might imagine that it’s expanding into a 4-dimensional space (impossible to visualize!) But there does not have to exist any such space. The expanding 3D space could be all there is. There doesn’t have to be anything beyond, from either a mathematical or physical point of view. This stuff is very hard to visualize because our brains and experience are 3-dimensional, and we’re limited by that. But it’s possible to get quite a bit of insight by analogy.
