Yes. Way, way, waaaay before the events of the films, a group of Jedi went to the Dark Side and got exiled. In their exile they found the Sith species (Sith isn't the correct name, but I can't be arsed to find it), a species that was basically the Dark Side incarnate. The exile Jedi and Sith species interbred, leading to the creation of the first Sith Empire. I'm pretty sure that the Sith species itself is extinct by the time of the films.