One theory that's gaining a lot of steam is that Snoke is Darth Plagueis the Wise. In Star Wars: Episode III — Revenge of the Sith, Palpatine tells Anakin the story of the fallen figure. Palpatine says, "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."
It's true, Palpatine goes on to say that Plagueis died. Even so, it's possible he faked his death. Or, given his extensive knowledge of the Dark Side, it's even more plausible he figured out a way to wrench himself from the grips of death. There's tons of evidence to support the theory, including a reasonable explanation, matching appearances, similar music, and more.
