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 as to 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. If you remember correctly, 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 season drops next year, watch this video to see if you believe the mind flayer is responsible for the emptiness of that world.