gluons

Gluons are elementary particles that mediate the strong nuclear force—the force that holds quarks together inside protons and neutrons, and binds those particles within atomic nuclei.

Unlike photons, which carry electromagnetic force but have no electric charge, gluons themselves carry the "color charge" they transmit, allowing them to interact with each other.

This self-interaction gives the strong force its unusual property: it grows stronger as quarks move apart, which is why quarks are never found in isolation.