Anthony Aguirre

The balloon metaphor is good but suffers from the flaw that people always ask about the inside and outside of the balloon, when you're supposed to imagine being a flatlander stuck on its surface.

Here's another way to think about a related version of a center-less universe. Consider the ancient game Asteroids, or anything more modern where the screen wraps around in both horizontal and vertical directions. That is, as you pass off the right edge of the screen you reappear on the left. Now imagine that screen filled with little tiny galaxies placed randomly. Where is the center of the distribution of galaxies? You might say the center of the screen, but in terms of what each galaxy 'sees' nearby, there is no difference between the center and any other place -- the galaxies don't see the edge of the screen, since photons just go from one 'edge' to another. In other words if you translate the entire thing to the right (or left or top or bottom) you don't change anything.

Now imagine a 3-dimensional space that wraps around in just the same way -- maybe in minecraft-world instead of asteroids.

This is a model of a flat universe with a toroidal topology. A finite closed universe is technically different, but this might help conceptualize things a bit better.