Most fans of Stranger Things understand the Upside-Down to be a parallel universe to Earth, but it doesn't quite explain why the world looks quite so desolate, cold, and gooey. But this video by Apollo Commander provides a theory about the source of the Upside-Down's gloomy atmosphere.
The fan theory explores the idea of multiverses, specifically the inhabitants of the Upside-Down. For those not caught up on Season 2, SPOILERS AHEAD.
According to the fan theory, the giant mind-flayer that torments and eventually possesses Will Byers is directly responsible for the bleakness of the Upside-Down. The flayer views itself as a superior organism, and desires to subjugate Earth to its power.
Hopefully, the Upside-Down will receive a better backstory with Season 3. But until the it drops in 2019, watch this video to see if you believe the mind-flayer is responsible for the emptiness of that world.