the universe is still expanding. We've known that since the 1930s, but that doesn't mean that it is expanding INTO anything. All we can actually measure (which is what astronomers substitute for speculation and philosophizing) is galaxies rushing away from us in all directions at ever-increasing speeds.

All expansion really means is that the distances between objects is increasing, but there is no wall or boundary that is being pushed out. Remember, no matter where you are in the universe, it looks like the universe is expanding away from you, and it looks to you as if you are at the center.

It's like a loaf of bread rising in the oven, the raisins are moving away from each other. Loaves have crusts, though, we can't see any, and we have no guarantee there is one. Or think of spots painted on a balloon. As the balloon inflates, the spots move apart, but none gets shoved up against or through a wall.

Its hard to visualize a 4-dimensional hypersphere, so we have to rely on 3-dimensional analogies, like an ant crawling around on the surface of a balloon that is being inflated. Of course, our analogy is mileading because real ants can tunnel into the balloon, or they can jump off its surface, i.e., they can actually travel into the 3rd dimension the balloon is inflating or deflating into.

But there is no way we can travel into a fourth dimension, because it is a mathematical concept, an idea we have formulated to do the bookeeping for us and allow us to do calculations. We can't even visualize it except by using mathematical formalisms, extending what works in 2-D and 3-D into 4-D, and even N-d; the equations still hold but the mathematical trick is not the real world, any more than the words we use to describe complex topics, actions and relationships the same thing as what they describe.

No, I do not understand it, either. All I know is that the universe does not really behave like balloons with ants crawling around on them. That model may help us understand it, even solve some problems and make some predictions numerically, but it is not the same thing. Whatever the universe is, it is under no obligation to behave like anything familiar to us. It does not have to make sense. Science only demands that it be consistent, and even that may only be an assumption on our part..