The Merchant said:

Heck if I had a crack at it, I would have made Palpatine a Dark-sider that only affiliated with the Sith for their knowledge and powers and took them for himself, combining it with Jedi and other Force teachings across the Galaxy. He would have been Dark Lord but once he had an apprentice he gave up on the title to them (Which would also explain why Vader is called Dark Lord of the Sith rather than Palpatine.)

There's nothing to contradict this in any of the six movies. In fact, I think that the OT supports this theory. Palpatine is never called "Lord" in the OT and the word "Sith" is never used in the OT, the latter suggesting that the Empire replaced the Sith.