May contain some spoilers read at one's desire
Recently there has been many speculation as to who the really ugly Snoke who's pulling the strings behind the first order. One such speculation is that Snoke is actually the mentor of Emperor Palaptine, Darth Palagueis. But before we get into the actual theory let's look at who he actually is.
The Greatest Sith Lord
Darth Palagueis was first mentioned in the movies in Revenge of the Sith when Palaptine was convincing Anakin to join the dark side.
"Darth Plagueis was a Dark Lord of the Sith so powerful and so wise, he could use the Force to influence the midi-chlorians to create life. He had such a knowledge of the dark side, he could even keep the ones he cared about from dying" - Palaptine to Anakin
Anakin, wanting to preserve the life of his love Padme, was finally swayed to the dark side. Even though Palaptine supposedly killed his master after learning all his secrets it could have been possible for him to trick Palaptine into thinking he was dead. Also the themes when played of Palaptine explaining to Anakin about Plagueis and the theme that plays in the background when Snoke is introduced are very similar but I won't post then here because that's not the point of this article.
Kylo Ren's call to the Light.
Now to the actual theory, if Palagueis is really alive then the balance supposedly brought about by Anakin when he killed the Emperor is not complete because a Dark Side force that is even greater than Palaptine is still alive. Which means that either Anakin was not the chosen one or Anakin himself is not necessarily supposed to balance the force. In a quote from JJ Abrams
"Kylo Ren idolizes Darth Vader, not Anakin Skywalker. He idolizes what Vader represents and what Vader was trying to do."
Darth Vader represents the complete order of the galaxy through power but not only that but the inner struggle of the light vs the dark which we see appearing in Kylo Ren's speech to his grandfather's old helmet. When he speaks of the "Call to the Light side" that "Master Snoke" can sense.
Close to the final scenes of the movie Han Solo in an attempt to bring his son back with him, he confronted Kylo Ren to have a heart to heart with him ultimately ending in his death. Now even though in the end Kylo kills his father you can see a slight struggle in his though process and he says he knows what he should do but doesn't know if he had the strength to do it which could mean one of two things. A- Going back to the Light even though he's been conditioned in the dark side or B-Killing his father. Obviously the one that actually happens is B but in both cases he still had a bit of conflict internally on what he should do.
Another point I have to make is at the end of the movie as the planet is imploding Snoke tells General Hux to collect Kylo and bring him to his base for the final part of his "training"
If Supreme Leader Snoke really is Plagueis then that would mean he knew all about the prophecy of Anakin bringing balance to the force. However, since Plagueis would not be dead total balance of the force would not fully achieved until he was dead. To stop this from happening Snoke turned Kylo to the dark side and trains him until he's completely under his control so that Kylo would not be able to turn on him such as Darth Vader did to Palaptine. However because of the call of the light Kylo will eventually turn on his master and finally bring balance to the force and bring an end to the cycle of apprentice killing master. Which would complete the prophecy of Anakin being the chosen one by directly influencing Kylo. Even if Palagueis isn't Snoke the rest of the Theory could be plausible just less likely.