A unified theory for all force?
The electromagnetic, weak and strong forces have much in common and are perhaps different aspects of a single force. They also appear to have the same strenght at very high energies, especially if supersymmetric particles exist. It may even be possible to include gravity if theories which treat matter as small vibrating string are correct.
The three quarks within the proton can sometimes appear to be free, although no free quarks have been observed. The quarks have a quantum mechanical property called colour an interact with each other through the exchange of gluons nature´s glue.
The process of electron-positron annihilation to quark pairs gives similar characteristic predictions, all these are also confirmed experimentally.The accumulation of many such results, where experiments match predictions based on quarks, convinces us that quarks are real.
People have long asked
"What is the world made of?"
"What holds it together?"