Our Universe is like a three dimensional version of the balloon’s surface, where galaxies are like raisins baking in a gigantic loaf of bread. (The bread is the invisible fabric of space; the galaxies are the raisins within.) We can measure the raisins within our view — where “our view” is determined by the speed of light and the amount of time that’s passed since the Big Bang — and we assume that there’s more raisins and more bread outside of what we can see, but that’s all we can know. We can determine the past expansion history of our Universe, we can find that the expansion is accelerating rather than any of the other options (and hence, describe the expansion), but as far as what’s happening outside of what we can observe, we have more questions than answers.

Are there four (or more) spatial dimensions, total? Is there truly a center to this loaf of bread (or to our Universe)? Is it infinite, or simply bigger than we can perceive? Does it ever curve back in on itself and reconnect? And is there something bigger and grander than what we can ever hope to observe, that it truly is expanding into? We not only don’t know, we have no idea how it would ever be possible to know. But that’s part of the wonder and joy of science: until we know, we have to admit that even the most absurd-sounding explanation that can’t be ruled out is actually possible. The Universe doesn’t need to be expanding into anything greater than itself; it may simply be expanding, because that’s what space does in general relativity. But it could be doing much, much more. If we’re lucky, perhaps someday we’ll devise a way to find out.