In response to a Quora question, I wrote the following:

This question was first answered successfully by Hideki Yukawa in 1935, who proposed that there is a nuclear force between protons (and neutrons) that is much more powerful than electromagnetism, but has extremely short range. In Yukawa's proposal, the short range was intimately tied to the fact that the "meson", the particle that mediated the force, was supposed to be quite heavy (in contrast, photons, which mediate the electromagnetic force, are massless). Yukawa's mesons, called pions, were confirmed experimentally in 1947, and he received the Nobel prize in physics in 1949. Shortly thereafter, it became clear that neither pions, nor protons and neutrons were truly "elementary" particles, as an ever growing zoo of elementary particles was being discovered in particle accelerators. Eventually, these were explained by the quark model, in which particles with fractional electric charge and a so-called "color" (not really color, just called so because it has three possible values, like human vision and the three primary colors) charge are held together by what is now called the strong force, acting on the "color" charge. Protons, neutrons, and Yukawa's mesons are all made up from different combinations of quarks. Yukawa's nuclear force, however, remains a valid phenomenological description for the force that binds nucleons together inside an atom.