I've already googled this question, and its not making sense to me. They keep trying to describe the universe as 4d, but the universe has 3 spacial dimensions that we live in, so how can it not be like a 3d expanding ball?? They use the surface of a 3d balloon as an analogy, but universe is 3D, no 4D. They are making it sound like the universe is 4d. Also, why can't we visualize 4d objects? If we lived in a 2d universe, we could look at a screen and visualize a 3d universe.