Another spin on this theory is that Luke Skywalker has become a Gray Jedi, and will use this new understanding of the Force to teach Rey. After the failure of his Jedi Academy, Luke has become disillusioned with the Jedi and their battle with the Sith. Perhaps he’s used his exile to study a different path. The prophecies of bringing balance to the Force, the underlying story in all the Star Wars films, could be true in this middle ground.
Additionally, given the definition of the Gray Force on Star Wars Wiki, “the ability to control the Gray Force was rare, as it required students to perform long hours (sometimes even days) of meditation”, it does sound a lot like Luke’s forced solitude.