Look, everything I said about Rey is speculation because there aren't enough hints... but this is fact. Snoke is Plagueis. There's no arguing it, there's more evidence here than against Casey Anthony.
The truth about Supreme Leader Snoke Darth Plagueis [EXTENDED THEORY] - YouTube
He went through all of the canon and found all of the evidence regarding Snoke's origins. Snoke said, "I've seen the Galactic Empire rise, and then fall." So he's very old and very powerful. If you watch his video, it leaves little doubt that Snoke is indeed Plagueis. Where else could have a powerful dark side user have received his training, since according to the rule of two it was Vader/Sidious? He has to be Palpatine's master, and there's even more canon lore in the novels to back this up.
Plagueis being a Muun is no longer canon, btw. There are also several other quotes providing evidence including that we know that Snoke had his power by the time of Ben Solo's birth, and that Palpatine may have been searching for him. His fucked up face looks like he survived some wounds that otherwise would not have been survivable had he not have the ability to, "create and sustain life."
"To cheat death is the power only one has achieved."
Snoke also tells Kylo that he's, "his most gifted apprentice." So he's trained other Dark Side users? Palpatine? So he's basically saying that Kylo Ren has more promise than Darth Sidious did, which is probably true because of Ben Solo's heritage as a Skywalker.
One last thing, Kylo Ren himself says in response to Han telling him that he'll be crushed by Snoke that, "The Supreme Leader is wise..." Wise? Dark Plagueis the Wise?
Between that reference, the theme, the quotes, and the fact that there can be no other explanation for such a powerful Dark Side user there leaves little doubt. It's canon that Snoke was sitting in the background watching Palpatine's machinations. Why else would such a powerful Dark Side user sit in the background and wait for Palpatine's fall if it wasn't his master? If Snoke isn't Plagueis, then where did he receive his training? Who else could he be?
Once you eliminate the impossible and whatever remains, must be the truth.
So to sum it up, Palpatine assassinated his master who was then resurrected by his ability to cheat death. By then Palpatine had achieved too much power and Snoke could not challenge him directly, so he went into hiding and waited for his former apprentice to fall. Then he came out of the shadows, a
formerSith more powerful than Palpatine, and began to turn Ben Solo to the Dark Side from the very beginning (his birth) exactly one year after the Battle of Endor. The Sith no longer exist, true, because he's not Darth Plagueis anymore. They and their teachings are gone, he's something else entirely, if not a very powerful adversary to Luke and Rey.