In The Phantom Menace, a prophecy is revealed that would seem to contradict previously-held assumptions about the relationship between the Light Side and the Dark Side -- that a Jedi will come who is destined to "restore balance" to the Force. We have seen in Return of the Jedi, and George Lucas has confirmed in interviews, that Anakin Skywalker fulfills the prophecy when he kills Emperor Palpatine and then dies himself, thereby bringing an end to the Sith.

When you picture the Light Side/Dark Side relationship as a Taoist yin-and-yang proposition this seems confusing. If the ideal state of the Force is one of balance, then surely the Dark Side must be present in order to balance the Light.

It is important to remember though that in the Star Wars series of movies the Light/Dark relationship is never actually described by any character as being one of interdependence or balance. Unfortunately, (or fortunately, given that we see these things to be entertained and not to sit through hours of Lucas' philosophical speculations) what information we do have about the way the Force works is sketchy at best. The novels, comics, and PC games based on the saga may expand on the subject, but Lucas does not regard these as canonical, describing them as an "alternate universe".

So we are never told exactly what there is within the Force to "balance", or how it can be knocked out of balance, only that the death of all known servants of the Dark Side is what is needed to restore it. I would suggest then that while the Yin-Yang imagery need not be discarded, the Dark Side should not be interpreted as an essential aspect of the Force that must be present in order for the Light to exist. Instead, it may be likened to the chaos and misery that results when the natural balance of things has been upset. Because the Sith have given themselves over to darkness, the Force has been disturbed, the natural powers of the Jedi are weakened and their thoughts thrown into confusion, and the once-harmonious Republic becomes an oppressive Empire.