One of the quantities described by Einstein's equations of General Relativity is the metric tensor—a matrix of matrices—that describes how "far" things are in s

One of the quantities described by Einstein's equations of General Relativity is the metric tensor—a matrix of matrices—that describes how "far" things are in space and time given the stuff in the local environment. One of the things that the equation tells us is that objects that are not gravitationally bound will tend to get farther away from each other as time passes. We call this the expansion of space. As far as we can tell, there isn't anything "outside" the universe is expanding "into"; distances just become larger somehow.