Please answer a question about the following article about Richard Feynman:

After the success of quantum electrodynamics, Feynman turned to quantum gravity. By analogy with the photon, which has spin 1, he investigated the consequences of a free massless spin 2 field, and derived the Einstein field equation of general relativity, but little more. The computational device that Feynman discovered then for gravity, "ghosts", which are "particles" in the interior of his diagrams that have the "wrong" connection between spin and statistics, have proved invaluable in explaining the quantum particle behavior of the Yang–Mills theories, for example, QCD and the electro-weak theory.

What did Feynman's discoveries help explain?
Yang–Mills theories