quarks are considered to be elementary particles, meaning they are not believed to be composed of smaller particles. According to the current understanding in particle physics, quarks are considered to be point-like particles with no known substructure.
However, it's important to note that our understanding of the fundamental constituents of matter is based on our current experimental knowledge and theoretical models. The field of particle physics is an ongoing endeavor, and new discoveries and theoretical advancements can always reshape our understanding.
Regarding the question of what the universe is ultimately made of, according to the Standard Model of particle physics, which is the prevailing theory describing the fundamental particles and forces, matter is composed of elementary particles such as quarks, leptons (e.g., electrons and neutrinos), and gauge bosons (e.g., photons and gluons). These particles interact through fundamental forces, including the strong nuclear force, electromagnetic force, weak nuclear force, and gravity.
However, the Standard Model is not considered a complete theory and leaves certain questions unanswered, such as the nature of dark matter and the unification of forces. There are several theories and models, such as supersymmetry, string theory, and various approaches to quantum gravity, that aim to provide a more comprehensive understanding of the fundamental constituents of the universe.
It is possible that future discoveries, advancements in experimental techniques, or new theoretical frameworks could reveal additional layers of substructure within particles or provide a deeper understanding of the fundamental nature of matter. But for now, quarks are regarded as elementary particles within the framework of the Standard Model.