In Volume 2’s “The Bridge,” it is revealed that the Upside Down is actually part of a wormhole connecting Earth and The Abyss, another dimension occupied by Vecna ( Jamie Campbell Bower ), the Demogorgons, Mindflayer, and more.

Stranger Things 5 Volume 2 rejected all the theories about the Upside Down so far. Steaming from Dr Brenner's Hawkins Lab, there's a lot more to the alternate dimension, which was explained by Dustin Henderson,

Stranger Things Season 5 Volume 2 introduces exotic matter as a key component of the Upside Down, but what is it, really?