To my amateurish understanding, it does go smaller than atoms, with subatomic particles like protons which themselves are made of quarks. Beyond that string theorists would say that there would be strings and that's it. Others might say some kind of quantum foam.

Going up, galaxies make up groups which combine into superclusters that form the cosmic web of the observable universe.

So for all intents and purposes, we can essentially say the universe and the various scales are infinite because it may as well be for humans.