Stranger Things is a popular show on Netflix set in the 80's following the lives of three young boys as they try to rescue their friend Will from "the upside down." Here is a clip of the boys science teacher explaining it a little:
Throughout the show, the upside down is described as a parallel universe, or an alternate dimension. However, as you dive deeper into the physics behind these concepts, there is perhaps a better way to describe the upside down. According to the following video, the upside down could be better described as a parallel reality. The video also dives into some crazy physics that explains how "the upside down" from stranger things could really exist. You can dive a lot deeper into the talk of other dimensions and universes and time travel and string theory, and the list goes on. But for now, I can appreciate the somewhat simplistic explanation that this video provides.
It's cool to analyze the science behind Stranger Things and dive into what's realistic about parallel realities. But I hope that demogorgon's are in fact fiction.