The concept of the "smallest thing" in the universe is a topic of ongoing scientific research and exploration. the smallest known fundamental particles are quarks and leptons, which make up the matter in the universe. These particles are considered point-like in the Standard Model of particle physics, meaning they have no size or substructure.
However, it's important to note that our understanding of the universe is continually evolving, and new discoveries could potentially challenge or expand our current knowledge. Physics beyond the Standard Model, such as string theory or other theories of quantum gravity, suggests the possibility of additional fundamental constituents or structures at smaller scales.
Regarding black holes, the singularity at the center is a region where our current understanding of physics breaks down. It is a point of infinite density and curvature, where the laws of physics as we currently understand them cease to be applicable. The singularity represents a limit of our knowledge, and it is not yet understood how quantum mechanics and general relativity might be reconciled to describe the physics at such extreme conditions.
In summary, the smallest known fundamental particles are quarks and leptons, but the search for a complete understanding of the ultimate constituents and structure of the universe is an active area of scientific inquiry.