Yay! but Huh?

I finished Final Fantasy 13 (late) last night and am overall very satisfied with the ending. This only seems to have about a 50/50 chance of happening with FF games. I understand the very very ending and how Fang and Vanille [spoiler]. It's the nature of the Big Bad that I don't understand. And how did Lightning et al suddenly know everything about it? That came out of nowhere. Maybe I would have understood if I'd done all the missions.