The unified form of the electromagnetic force and the weak nuclear force. The electroweak transition is at a low enough energy (on the order of 1 TeV) that, IIRC, high-energy physicists were able to experimentally verify it. The formal acceptance of the electroweak force theory resulted in the creation of the standard model of physics that we rely on today. See the Grand Unified Theory node for the attempts to unify this force with the strong nuclear force. One prediction of electroweak theory that remains missing-in-action is the elusive Higgs boson.
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