The concept that quarks are elementary particles, meaning they are not made up of smaller constituents, is based on our current understanding of particle physics. According to the standard model, which is the prevailing theory describing elementary particles and their interactions, quarks are considered fundamental particles.
In the standard model, quarks are considered point-like entities, meaning they do not have a size or substructure. They are treated as elementary because they are not composed of any smaller particles or entities. Quarks are considered the basic building blocks of matter, along with other elementary particles like leptons (such as electrons) and gauge bosons (such as photons).
Now, it is important to note that our understanding of the fundamental nature of particles is based on experimental observations and theoretical models. These models have been successful in describing a wide range of particle interactions and phenomena, but they do not provide a complete picture of the fundamental nature of particles.
It is possible that at even smaller scales or at higher energies, our current understanding may need to be revised, and new substructure or constituents may be discovered for particles like quarks. This is an active area of research in particle physics, and experiments at high-energy particle accelerators, such as the Large Hadron Collider (LHC), aim to probe the structure of matter at smaller scales and explore the fundamental constituents of particles.
So, while our current understanding considers quarks as elementary particles, it is always possible that future discoveries and advancements in our understanding may reveal a more intricate substructure to these particles. Science continually evolves as we gain new knowledge and insights through experiments and theoretical advancements.