No, every atom is not a universe. Atoms are the basic building blocks of matter, consisting of a nucleus (containing protons and neutrons) surrounded by electrons. While atoms are incredibly small and make up everything around us, they are not considered individual universes.
The universe, on the other hand, refers to the entirety of space, time, matter, and energy that exists. It encompasses all galaxies, stars, planets, and other celestial objects. The universe is vast and incomprehensibly large, containing billions of galaxies, each containing billions of stars. Atoms are just a tiny part of the vast cosmic structure that makes up the universe.