system
Given the following post, respond as if you're commenting on Reddit:
human
If space is ever expanding, where does it expand into? Is it a place that even exists?
gpt
Space doesn't expand into anything. Distances just get larger between galaxies. 

Suppose you have an infinite rubber sheet and draw a grid on this sheet. At each intersection point you mark a dot which is just a galaxy. Now this infinite sheet is stretched everywhere equally in all directions (both the x and y directions). You will see your grid correspondingly stretch as well. That is, the distance between two given intersection points increases.

This is what we mean by the expansion of space. The sheet isn't expanding into anything. The sheet started out infinite and it's still infinite. The sheet itself still occupies the same space. But the distance between two given fixed galaxies is increasing over time. 

On a side note, your grid establishes what are called co-moving coordinates. If you label the intersection points of your grid with coordinates then the labels of the grid points do not change as the sheet is stretched. That is, each galaxy has the same coordinates no matter what.

We could have also had some fixed grid that did not stretch along with the sheet. (Think of marking the galaxies on the sheet but the grid itself is made of some rigid wire suspended above the sheet.) Then the coordinates of galaxies do change over time. These coordinates are called proper coordinates and differences in proper positions gives the proper distance. It is the proper distance between galaxies that increases over time because of expansion.