We had a marvellously accurate theory of electromagnetic forces, quantum electrodynamics, or QED, a quantum version of Maxwell's theory. The weak forces are due to the exchange of W and Z particles. However, the origin of their masses remains mysterious. The fundamental theory exhibits a beautiful symmetry between W, Z and photon. But this is a spontaneously broken symmetry.
Spontaneous symmetry breaking is a ubiquitous phenomenon.
For example, a pencil balanced on its tip shows complete rotational symmetry - it looks the same from every side. The pencils are all coupled together, so that they all tend to fall in the same direction. Their presence in the vacuum influences waves travelling through it. The theory can be tested, because it suggests that there should be another kind of wave, a wave in the pencils alone, where they are bouncing up and down. That wave is the Higgs particle.