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"The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale."

https://en.wikipedia.org/wiki/Expansion_of_the_universe

This was confusing for me too. As time passes, the less the universe is made up of "something" (observable matter), so maybe a more accurate analogy would be saying the universe is "diluting" rather than "expanding"?

My understanding is along the lines of, the surface of a balloon is the "fabric" of the universe: As the balloon is blown up it can expand and push things on it further apart, but the matter itself is always in the same place on the surface/in the universe (or moving slowly along the surface/in the universe). One big difference of course being the surface of a balloon is 2-dimensional, while space is 3-dimensional.