Possible spoilers for Star Wars: The Force Awakens ahead!
Also, mention of midichlorians ahead - I know that hurts peoples' eyes.
A popular theory is circulating the internet right now, and it centers on the mysterious identity of Supreme Leader Snoke in Star Wars: The Force Awakens. The character is imposing in the film, and people are clearly scared of him. General Hux has a fanatical devotion to him, Kylo Ren regards him as a revered master, and Han and Leia speak of his power, specifically his ability to seduce people into the dark side.
But what do we really know about him? We don't know what his real looks and size are - assuming his face is actually the scarred mess we see in giant hologram form, then that's about the only thing we know about him for sure.
Well, thanks to the musical cues in The Force Awakens and one very similar one in Revenge of the Sith, some fans think they may have figured out the mystery of Snoke's identity. When Palpatine, who is of course also Darth Sidious, tells the story of his master Darth Plageuis to Anakin Skywalker, a low-pitched, slow monk refrain sings in the background. A very similar one also happens to be "Snoke's Theme" in The Force Awakens.
It's not definitive, but it's absolutely a clue that makes sense. The team at Lucasfilm, J.J. Abrams, and John Williams are all known for the way they slide clues into even the smallest things - and also for being very careful with this sort of choice. Williams weaves important musical cues into all his work, especially within Star Wars soundtracks. A hint of the Imperial March here, a touch of the Force Theme there, and you hear the future of these characters emerge.
Plaguies is himself a whole other mysterious can of worms, though. One of the last novels published before things switched into Disney's new official canon, Darth Plagueis is now longer considered part of continuity as it's been moved into the "Legends" category, so it's hard to look there for any other clues. There have been some other options to Snoke's identity suggested by fans reading every last bit of the Star Wars stories that are canon, too.
But those are theories for another post.