I believe one of the Timothy Zahn novels sheds light on the Dagohbah cave. Yoda using a place heavy in darkside vibes to conceal himself from the Empire makes more sense than simply choosing a planet off the beaten path. I guess as far as official explanations go, we're always going to wonder what that place was before it was overgrown. You can clearly see a wall in one shot, so it's not a natural cave,