In the trailer for The Last Jedi, Luke Skywalker mentioned that there was something bigger happening in the Force than just light and dark. I have been pounding this question against my head for a while and I have yet to receive a definitive answer. Here is what I think so far. The light and dark sides of the Force are two ends of a spectrum. While the light and dark sides are VERY familiar territory for Star Wars fans, the area in the middle is largely unexplored territory. Even in the Legends continuity, the middle of the Force was very mysterious and gave us only brief glimpses of its nature and power. In the new Canon, we got our first wielder of the middle of the Force: the Bendu, who was an embodiment the balance between light and dark. I think Luke Skywalker may have found something in the First Jedi Temple that will enable him and Rey to access the light and dark in perfect balance. This may also shed light on why Luke said the Jedi had to end because Luke may believe that accessing the light side alone is not enough to defeat Supreme Leader Snoke. With this in mind, I think The Last Jedi will give us a clearer picture of what the middle of the Force is like. I hope I will receive an answer this December.