In the current understanding of particle physics, the smallest known fundamental particles are considered to be elementary particles. These particles are indivisible and do not have any internal structure. The elementary particles currently known include quarks, leptons, gauge bosons, and the Higgs boson. These particles are point-like, meaning they are treated as having no size or spatial extent.
The existence of elementary particles is supported by a combination of experimental evidence and theoretical frameworks such as the Standard Model of particle physics. Experimental particle accelerators, such as the Large Hadron Collider (LHC), have been used to probe the fundamental constituents of matter and have discovered and studied various particles.
While we can indirectly observe the effects of elementary particles through their interactions and the particles they produce, we cannot directly observe them in the conventional sense. This is because their size is so incredibly small that it is beyond the reach of current experimental techniques. The scale at which elementary particles operate is far smaller than the resolution of any existing microscope or imaging technology.
Instead, scientists rely on high-energy particle collisions and sophisticated detectors to study the behavior and properties of elementary particles. By analyzing the patterns of particles produced in these collisions and measuring their energies, momenta, and decay processes, scientists can infer the existence and properties of the elementary particles involved.
It is worth noting that our understanding of fundamental particles and their behavior is continually evolving as new experimental evidence is obtained and new theoretical frameworks are developed.