So it's quite possible that other people stumbled on it before Qui Gon.
The Dark Side being stronger are Lucas's words, not mine. Is he saying Yoda is wrong? Or is it something like the strength of the Darkside waxes and wanes... maybe sometimes it's actually stronger and holds more sway in the galaxy. It's not like in these stories the underdog can't win. Even if the Dark Side is a corruption and abnormality, that can still makes sense if it holds sway for a time.