I have a theory. If its already been thrown out there apologies, but I haven't seen it yet.
Snoke is... Luke. Luke is Snoke.
We're supposed to believe Luke has been off in isolation all these years, ashamed, because he couldn't control Kylo, who took out a temple full of Jedi recruits? The same Kylo that could barely hold his own against a reformed storm trooper who picked up a lightsaber for like the 2nd time ever? This, after years of honing his abilities since the slaughter of the Jedis?
Nah, son. Kylo had help.
Ever since the beginning Luke has had dark forces stirring inside of him. The entire premise of the first films was the battle on both a galactic scale, and an individual scale, within Luke. Vader/Emperor thought until the very end that they could turn him. That internal tug between good and evil was never truly resolved, and even now it continues. Luke, at some point, possibly sparked by his struggles with Kylo's ultra radical dark inclinations, snapped. He did something very, very bad. He gave in to the same forces that almost caused him to kill his father in ep VI. Perhaps he allowed Kylo to slaughter the padawans, perhaps he directed him to, perhaps he did it himself. But he realized what he had done, surely was ashamed, and saved the (one? last? super powerful/special?) remaining padawan by whisking her away to grow up on a desert-like planet, just as he did.
Knowing that he must first defeat once and for all the dark side within himself, he retired to an uncharted planet to live as a hermit and confront Snoke, the dark version of himself. No one must know where he is, because it would be too dangerous for anyone else to confront him, Luke knows its up to him and him alone. He has been battling Snoke for years, but Snoke's power grows, and he directs the First Order through a holographic avatar from this unknown planet.
Luke did leave one piece of insurance with his trusted droid R2D2: should the last padawan turn up, Luke's location would be revealed as she would be the only remaining hope of defeating Snoke. Luke must now decide if he can train her to do just that, and thus sacrificing himself for the good of the galaxy...
