Interesting ideas whirling around in here...
I was thinking... maybe we should put not the Force into context, but the users of the Force. The Force itself has a flipside, to be sure. But the element itself I don't think is good or evil. Its uses and the intent behind such uses become good or evil. That said, if balance is the use of the Force that is harmless, the reverse would be imbalance, the use of the Force causing harm, directly or indirectly.
So the idea of imbalance isn't the existance of evil; evil is something that has to exist for good to be detereminable. The idea of imbalance is the existance of the Sith, the opposing team to the Jedi, who (generally) represent order. It -is- called a Jedi Order, isn't it? (Word play there... )
Now, we have this notion of power. What is power? It is the ability and intent to do something according to your will. It is the ability to change minds, or cause storms, or even ground your kids to their rooms. We could easily say, for the sake of argument, that the Dark Side (tm) is stronger. Certainly it appears that way. But what is the end to which this power is a means? Evil is self-destructive. It does not create lasting institutions... no empires, no Orders... certainly no semblances of trust. Therefore it is self-defeating. The power that can be attributed to the Light Side of the Force, the one that regards balance and order, is a power of making lasting achievements. It can only be defeated by stagnation, I suppose. Perhaps the imbalance of the Force was not due to the reemergance of the Sith, certainly since there were only two Sith in existance, working secretly. In the past, whole legions of Dark Jedi existed. And no prodigy was born then. Soooo... The imbalance must have come about through the stagnation of the Jedi Order. Perhaps their teachings had failed in some attempt, or did not adapt to the change or temperment of the times.