Final answer:
The discovery of W and Z bosons as predicted by the electroweak theory provides strong evidence for electroweak unification. Quantum chromodynamics describes the confinement of gluons and quarks within hadrons, explaining why they cannot be directly observed.
Step-by-step explanation:
Electroweak unification is supported by substantial evidence, particularly through experimental observations of weak force interactions and the prediction and discovery of electroweak bosons. One standout moment in demonstrating this unification was the discovery, in 1983 at CERN, of the W and Z bosons, which was a major prediction of the electroweak theory developed by Steven Weinberg, Sheldon Glashow, and Abdus Salam in the 1960s. They predicted the existence of these particles and their masses to be around 81 GeV/c² for the W bosons and 90 GeV/c² for the Z boson, which matched the experimental observations. While the strong force, carried by gluons and described by quantum chromodynamics (QCD), is not yet unified with the electroweak force at energy levels that can be readily tested, its unification represents a goal for Grand Unified Theories (GUTS). As for gluons and quarks, they are both confined within hadrons and cannot be directly observed due to the color charge they carry and the strong force. The absence of free quarks and gluons despite high-energy collisions provides indirect evidence for their existence and confinement.