Yeah, the exact definition of "balance" is unclear. My take on it is that by the time of
AOTC and
ROTS, the Sith clearly have the upper hand. The Jedi's ability to use the Force has been weakening as Sidious becomes more powerful than Yoda or the rest. Along comes young Skywalker, perhaps the post powerful Force senstive ever, who can equal Sidious (at least until OWK slices him into cold cuts and then DV is less powerful). Sidious, of course, senses what the Jedi don't, and lures Anakin over to his side, throwing the Force way off balance because instead of one powerful good being and another powerful bad being striking a balance, there are two baddies.
When Anakin kills the emperor and dies himself the balance is restored. There clearly must be other Force sensitives throughout the galaxy, some good, some bad. Whether untrained people can sense it or not, The Force is everywhere (remember the
ESB cave, which Yoda said is powerful with the Dark Side. It's just a cave, but the Force is there).
We can assume Luke revives the Jedi religion, but you can also assume out there somewhere the Sith still lurk too.
You could also look at it that the Force, like nature, itself knows when it is out of balance, hence the creation of Anakin by the midiclorians to return the balance.
So perhaps you could say that Sidious, who comes before Anakin, is the one who unbalances the Force and Anakin rebalances it.
Or you could say that we're an awful lot of geeks.
