The concept you are referring to is known as quark confinement, which is a fundamental principle in particle physics. Quark confinement explains why isolated quarks have never been observed in experiments. Instead, quarks are always found in bound states called hadrons, such as protons and neutrons, which are composed of combinations of quarks and antiquarks.
Quark confinement is a consequence of the strong nuclear force, also known as the strong interaction or strong force, which is responsible for binding quarks together inside hadrons. The strong force is characterized by the exchange of particles called gluons. Unlike the electromagnetic force, which weakens with distance, the strong force between quarks increases as they are separated. This means that the energy required to pull quarks apart increases as the distance between them increases.
As the separation distance increases, the energy stored in the strong force eventually becomes so large that it is energetically favorable to create a new quark-antiquark pair from the vacuum. This process is known as quark-antiquark pair production or quark pair creation. The newly created quark-antiquark pair combines with the original quarks, forming new hadrons. In this way, quarks are "confined" within hadrons and cannot exist as free, isolated particles.
The conservation of matter is not violated because the newly created quark-antiquark pairs compensate for the separation of the original quarks. The total number of quarks and antiquarks remains conserved throughout the process.
Quark confinement is a fundamental feature of the strong force and is a well-established concept in the theory of quantum chromodynamics (QCD), which describes the interactions of quarks and gluons. While the precise details of quark confinement are complex and still an active area of research, the overall principle is widely accepted within the scientific community.