It's been known since the 1970s that there are three 'generations' of quarksplugin-autotooltip__plain plugin-autotooltip_bigQuarks

Following theoretical calculations, particle accelerator experiments in the 1970s confirmed that quarks are responsible for the internal structure of protons and neutrons. ( 3 quarks each, in different configurations, see :Electron / Quark charge balance).

Again, as calculations predicted, six 'flavours' of quark have now been experimentally identified (ref. (the building components of neutrons and protons) and also leptons (electrons and similar particles).

All the particles have been rigorously confirmed in multiple experiments, and the three 'generations' (possibly better understood as 'families') are now firmly part of the Standard Model of nuclear physics.

^{[ table from Wikipedia ]}

There is currently no explanation as to why there should be three distinct generations of each.

Several experiments have attempted to find fourth generations of various particles, but none has yet been found (example research from CERN).