The Force

Kid, I've flown from one side of this galaxy to the other. I've seen a lot of strange stuff, but I've never seen anything to make me believe there's one all-powerful Force controlling everything. No mystical energy field controls my destiny - it's all a lot of simple tricks and nonsense.
Han Solo in A New Hope

In the Star Wars universe, the Force is an energy field produced by all living things, enabling certain people with psychic powers such as telekinesis and precognition. The true nature of the Force is never fully explained, although it is reduced to technobabble by the introduction of Midichlorians in later movies.

The Force can be used in different ways, for good or evil; the Force has a two sides, light and dark. The light and dark sides of the Force are the Yin and Yang of Star Wars. A Jedi uses the Force for knowledge and defense, never for attack (although the Jedi Mind Trick is acceptable). Fear and hatred are the allies of the Sith, followers of the Dark Side.

Which side is stronger? Jedi Master Yoda's point of view is that the dark is not:

YODA: A Jedi's strength flows from the Force. But beware of the dark side! Anger, fear, aggression - the dark side of the Force are they. Once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan's apprentice.

LUKE: Vader. Is the dark side stronger?

YODA: No. Quicker, easier, more seductive.

Whether this is true, or if the light side is any stronger than the dark side, remains to be seen. Battles with the force and lightsabers between Jedi and Sith have had varied outcomes. Furthermore the light and dark distinction could be meaningless construct - as Hamlet says, there is nothing either good or bad but thinking makes it so.

May the Force be with you.

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.