Once you defeat the Heart of Darkness, the fog lifts and you find out your on Mars (your Ghost says so and the ceiling turns to Mars red). There is a grimoire card that describes a Dyson sphere that looks like the image Spectre added. Also if you think maybe maybe the Traveler is a Dyson sphere then you're wrong because it is too small to support a small star, or it uses space manipulating technology.

Ah, yes. I remember that, now. So much for my idea! Perhaps the garden was corrupted through the misdeeds of the Races--like a Garden of Eden allegory? Or perhaps the story will be based on the Eastern conception of Yin and Yang--that the Darkness and the Light are in symbiosis, always contending but destined to collapse and give rise to the other?