This was discovered in 1967-1971 by Steven Weinberg, Sheldon Glashow, and Abdus Salam. They got the Nobel Prize in physics for unifying those forces. Finally I am ready to talk about the strong force. These quarks have strong charge and are bound together by the strong force. The particles which transmit the force are called gluons.
Gluons are massless, like the photon.
But unlike the photon, which is electrically neutral, the gluons carry strong charge and a different strong anti-charge. Because the gluon is massless, at first you might think the range of the strong force is infinite, like the EM force. But if you study the behavior of the strong force, you find that the three quarks in a proton or neutron behave almost as if they were bouncing around freely in a relaxed, elastic spherical container. None of the quarks can escape the container because when the quark reaches the boundary of the proton or neutron, the force begins to act and gets stronger and stronger the further away the that quark gets from the others. That is very different from the other forces which get weaker at longer distances and it occurs because the gluons have the color and anti-color charge.