Oh. Doh! I kept on thinking about the "First Cause" because that's usually what all these discussion are about. You were asking about what kept the expansion going after... I'm an idiot. I admit! You kept on saying it, and the coin never fell.
Ok. Got it.
The energy that adds to the inflation as of now. Right. Yeah. The current scientific idea is dark energy (if I understand it right). There's nothing else that can explain it as of now. Also, if I'm not completely wrong, dark energy and dark mass aren't really part of the Big Bang theory at all. No one knew about them until recently.
The way I imagine how the universe works is rather more like an ocean of water where someone drips a drop of water occasionally here or there, like raining a slow rain. Each drop creates a set of waves. But in the universe sense, each drop is a spot of dark energy. Something like that. I think that there are constant input of more energy into the universe. And the "Big Bang" event that we see in the past is more of our own matters first occurrence in the universe. We see it as a point in time of a "bang" but in reality, there never was a beginning of the total universe. Not sure if that makes any sense, but it's okay if it doesn't. Can't explain it anyway.
Got it. My bad.
I got stuck in thinking of the overall discussion in this thread and not the specific question you had.
I was about to call you an idiot, but finally I realized it was me.