Quarks are elementary particles and are currently considered to be fundamental constituents of matter. They are not composed of smaller particles or substructures. Quarks are the building blocks of hadrons, which include protons and neutrons. There are six known types of quarks, known as flavors: up (u), down (d), charm (c), strange (s), top (t), and bottom (b).
Quarks are never found in isolation due to a property known as color confinement. This means that quarks are always bound together in groups to form particles called hadrons. Hadrons can be either baryons (such as protons and neutrons) composed of three quarks, or mesons composed of a quark-antiquark pair.
Although quarks are considered fundamental, the search for even more fundamental particles continues in the field of particle physics. For example, scientists are exploring the possibility of substructure within quarks or the existence of other particles beyond the Standard Model, such as hypothetical particles like preons or compositeness. However, no experimental evidence has been found to suggest that quarks are not fundamental. It's important to note that scientific understanding is always subject to revision and new discoveries, so future research may shed more light on the fundamental nature of quarks or reveal new particles and interactions.