Quarks are elementary particles that are typically found in groups of two or three, bound together by the strong nuclear force to form composite particles such as protons and neutrons. This phenomenon is known as confinement. Due to confinement, isolated quarks are not observed in nature.
The strong nuclear force, which is responsible for binding quarks together, becomes stronger as quarks are pulled apart. Consequently, the force between two quarks increases with distance until it is energetically favorable to create a quark-antiquark pair or additional quarks to form color-neutral composite particles.
This behavior of quarks is described by a theory known as quantum chromodynamics (QCD), which is part of the standard model of particle physics. Quarks are considered to be permanently confined within composite particles, and their individual properties cannot be directly observed or isolated in an experimental setting.
So, while quarks exist as fundamental particles, they cannot be found independently or isolated from other quarks due to the strong force that binds them together.