The current understanding in particle physics is that quarks are considered to be fundamental particles, meaning they are not composed of smaller constituent particles. According to the Standard Model of particle physics, which is our current best description of elementary particles and their interactions, quarks are considered to be point-like particles with no internal structure.
Quarks come in different types or "flavors," such as up, down, charm, strange, top, and bottom. Up and down quarks are the lightest and most commonly encountered quarks in ordinary matter. They combine to form protons and neutrons, which are the building blocks of atomic nuclei.
The properties and behavior of quarks have been extensively studied and measured using high-energy particle colliders like the Large Hadron Collider (LHC). The LHC has provided significant experimental evidence supporting the notion that quarks are indeed fundamental particles.
However, it's important to keep in mind that our understanding of the fundamental nature of particles is constantly evolving. While quarks are currently considered fundamental in the Standard Model, it is possible that future scientific advancements may reveal a more intricate structure or underlying constituents. It's difficult to definitively rule out the existence of any substructure for quarks without experimental evidence, so it remains an open question for further exploration and investigation. Nonetheless, as of now, there is no direct evidence or theoretical indication that quarks are composite particles made up of smaller constituents.