system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If before the universe there was nothing, then the universe was created, shouldn't there be a field of nothing at the possible edge of the universe?
gpt
A common misconception about the expansion of the universe is to imagine it's expanding in all directions in three dimensions. You envision a balloon that we're inside. The balloon is getting bigger, and you say 'there must be an actual balloon around this space'.

In reality, the universe as we know it *is* the surface of that balloon. The balloon is expanding into the fourth dimension. Like two dots on an inflating balloon, any two objects in the universe that aren't locked together are getting farther and farther apart. The speed of this separation is always proportional to the distance between the dots, with more distant dots moving away faster. Space is essentially being created everywhere at once, from our perspective.

Because of these observations, we know that the universe is expanding in the 4th dimension. But we have no idea what that looks like beyond that. Maybe, somewhere, not up, not forward, not left or right, but some fourth direction perpendicular to all of these there is a great wall of the universe. But we can't see it.

In the three dimensions we perceive and move in, the universe is boundless, like the surface of a ball.