The universe may be both expanding and closed.
The Universe is expanding. We have ascertained this by realizing the stars on all sides of us are red-shifting. This means that things are getting farther apart. But, there’s a very real possibility that it’s not expanding into anything. In fact, the Universe may very well be a closed-system. Us and the stars we observe may be like dots on the surface of a balloon as its blown up. They spread apart, but if you trace a straight line for long enough, you’ll end up back where you began.