I used to wonder this quite a bit. Just how old was the good Senator before we see him in episode-1?

I was never really into the EU too much. I think that Lucas intentionally leaves a lot of room for the imagination within his audience. While it is nice to hear what some of the other fans have thought of, I like to use the motion pictures as my basis for imagination. I have, since I turned to the "blog-side" a few weeks ago, been exposed to more EU than I had in my whole life previously. Its like I just have this internal hunger to hear 'the rest' of the story...

Anyway, back to the topic: I have learned now from the EU that there were many, many Sith during the 1000 years that they were thought to have "disapeared." Here is the theory that I thought up before I learned any of the EU stuff about the Sith....

Go back 1000 or so years before Episode 1. The Sith had reach an Apex in the developement of their ability to control the Dark-SIde of the force. They had become so powerful, that they were beginning to gain "un-natural" abilities that where borderline Super-natural, or even "Divine" depending on your point of view.

The Jedi-Order was faced with a hard decision. The Sith had moved beyond the point of being evil Dark-Siders, and had become an abomination to the balance of the force. The "un-natural" things that they were doing were tearing at the very fabric of the force. Not only the Jedi Order, but also the Dark-Jedi (funny that I thought of the idea of Dark-Jedi on my own, it seems to be a popular theme in the EU) were disturbed by the actions of the Sith. While they were not necesarily in league, all the Jedi converged upon the Sith in an attempt to bring the Force back into balance.

As the Sith were being destroyed, the Jedi could feel that the Force was coming back into balance. The Fabric of the Galaxy was not repaired, however, but only mended shut.

Somehow, a lone Sith survived and found a way to draw energy from the "wound" that had been created, and continued to perform his abominations undetected by either the Jedi Order or the Dark-Jedi. Slowly, over the course of the next 1000 years, this wound grew.... ever so slowly. Snaking its way back into the fabric of the Universe undetected.

The Sith had become so powerful that they could manipulate the midi-cloreans that create life. They could essential live forever by training an apprentice to the point in which they are able to destroy the soul of the apprentice and take over the physical matter that makes up his body. It was a tricky thing, however. The Sith had to train his apprentice to the point where he was adept enough to have the skill to do this himself, and the Sith Master had to recognize when the time to make the jump was right. If the Aprentice became adept without the Master realizing it, the Apprentice would kill his master so that he would be the one to live on forever. Perhaps there were 20 or 30 Sith Lords over the course of 1000 years, but maybe in actuality, there were only 4 or 5, as the Master usually defeats the aprentice.