Warning: possible spoilers ahead!
Every fans seems to be focused on the mystery behind Luke Skywalker's story in Star Wars: The Force Awakens, but let's not forget that there are other mysteries that the film will probably unveil, such as Andy Serkis' motion-capture role as Snoke. No clues about his character has been revealed in any of the trailers, TV spots, or promotional posters, but some fans believe that his real identity is Darth Plagueis, whose greatest contributions to the history of the galaxy was training Darth Sidious in the ways of the Sith and the dark side of the Force, whom he incited to take control of the galaxy and bring about a new age of the Sith.
YouTuber The Stupendous Wave put together a follow-up video to this intriguing theory to present the details from concept art, previous story details from Expanded Universe novels, and possibilities on how he had escaped death. I hope that Snoke will actually turn out to be Darth Plagueis, and I guess we'll have to wait for The Force Awakens to get our answers, but for now, watch this video that explains this theory.