Major GL spoilers, naturally.
There are a few widely-accepted speculations among those who have discussed this at length: the beginning was one of the "dreams" induced by the antispirals near the end of the series; alternatively, it was intended to actually be foreshadowing of something, but that changed when the director changed after the fourth episode and it was simply forgotten; or, it is simply there to be a demonstration of what happens when, as the antispirals warned, spiral power is misused, since "Great Dimensional Government" sounds like a collection of human worlds, and Simon, ignorant of the dangers of his spiral power, uses them to destroy the universe.