According to our current understanding of particle physics, the smallest known particles are called elementary particles. Elementary particles are fundamental particles that are not composed of anything smaller and are considered indivisible.
The Standard Model of particle physics describes a set of elementary particles, including quarks, leptons, gauge bosons, and the Higgs boson. Quarks and leptons are believed to be point-like particles with no internal structure. These elementary particles are considered fundamental because they do not appear to be composed of anything smaller or divisible into smaller constituents.
The reason we haven't found any smaller particles or substructure within elementary particles is because our current experimental capabilities have limitations. Particle accelerators, such as the Large Hadron Collider (LHC), have provided us with valuable insights into the subatomic realm, but they have their technological and energy limitations. As our understanding of particle physics evolves and experimental techniques advance, it is possible that new discoveries or evidence of substructure within elementary particles may emerge.
Furthermore, it's important to note that our current understanding of particle physics is based on the best available scientific theories and experimental evidence. New discoveries and advancements in the field of particle physics may lead to revisions or updates in our understanding in the future.