According to our current understanding of physics, atoms are considered the fundamental building blocks of matter in our universe. However, it is important to note that the size of atoms is not an absolute quantity, but rather depends on the scale at which they are observed.
In the context of our observable universe, atoms are not considered to have substructure. They are typically composed of a nucleus (consisting of protons and neutrons) surrounded by electrons. The size of atoms is determined by the distribution of their electron clouds and is on the order of a few angstroms (10^-10 meters).
That being said, it is theoretically possible that there could be universes or physical systems with different fundamental particles or substructures smaller than what we currently observe. Speculative theories, such as string theory, propose that particles may be made up of tiny vibrating strings or other fundamental entities. If such theories are correct, it is conceivable that there could be universes or physical systems with subatomic constituents smaller than our atoms.
However, it's important to emphasize that these ideas are currently speculative and remain areas of active research and theoretical exploration. Our understanding of the fundamental nature of the universe is still evolving, and it is challenging to make definitive statements about the properties of hypothetical universes beyond our own.