In ancient times, the Jedi warred among themselves. A terrible conflagration which spread across the known galaxy as the followers of Light and Dark sought to drive one another into extinction.
In that time, the Light prevailed.
The followers of the Dark Side of the Force fled to the outer reaches of the galaxy into uncharted space. They settled new worlds and enslaved the primitive races which inhabited them, creating a dark empire. They called themselves the Sith.
The Sith sorcerers lived apart from the Old Republic, gaining strength and numbers while consolidating their power. Dark Lords rose to power and died by the dozens during their exile, such was its length.
Then, fate brought them back to the known galaxy, to the Old Republic, where they fell again upon the followers of Light. But this time it was different. Over the centuries the dark magic of the Sith had become even more twisted by the savage frontier worlds which they colonized. The powers of Darkness where equal to that of the Light....
And the battle raged. Light fell to Dark and the Dark to the Light. Decades passed and the Republic acclimated itself to the presence of the Sith among them, spreading dissent. The number of Jedi dwindled with those of the Sith as the war took its toll upon their ranks.
Today the true blooded Sith are no more. Their teachings live on though, as does their bloodline, thinned throughout the galaxy with generations of interbreeding, but still the Darkness lives on.