Ever since the release of Stranger Things Season 1, fans have been theorizing about the true nature of the Upside Down. Some theories suggested it was an alternate dimension filled with monsters and terrifying creatures, while others indicated it was Hawkins itself years into the future after a nuclear fallout. Finally, some proposed that the mysterious place was simply a bridge to somewhere far more dangerous. Well, as it turns out, this last option was right on the money.
Stranger Things Season 5 Episode 6 unveiled the truth: the Upside Down is a wormhole—a fissure in spacetime that connects Hawkins (and only Hawkins) directly to another world, which Dustin calls The Abyss. That's why the Russians weren't able to open a gate to the Upside Down from their home country and had to conduct experiments in Hawkins instead. It also explains why the mysterious place is stuck in time on November 6, 1983, as it is the date it was first opened. However, there's one specific topic that hasn't been explained all that well, and that is exotic matter.
Right on the rooftop of the Hawkins National Laboratory in the Upside Down, Nancy and Jonathan find what is explained to be exotic matter back in Episode 5. When they shoot it, all sorts of wacky things start to happen. The floor beneath their feet begins to melt, and waves of energy traverse the sky until they hit the circular wall enclosing the Upside Down. So what is this mysterious exotic matter, and what role does it play in the world of Stranger Things?
Exotic matter is the key ingredient to keep a wormhole open
For starters, it's worth clarifying that exotic matter is just a theoretical concept, as it hasn't been observed in our real universe just yet. But if it were to exist, it could serve to keep a wormhole open and stable. See, if an Einstein-Rosen bridge were to form, chances are it would collapse so quickly that not even light could traverse it before being closed once again. As a result, there needs to be something at the "tunnel" of a wormhole, pushing it from the inside out, to keep it open for longer periods of time. Well, that's where exotic matter comes in.
Exotic matter has the particularity of possessing negative mass (or, at least, that's what the theory dictates). In the world of physics, that means that instead of exerting a gravitational pull, it provokes a gravitational push. In other words, exotic matter is the one thing keeping the wormhole alive as it keeps the throat of an Einstein-Rosen bridge open. The circular wall everyone sees in the Upside Down is constantly trying to collapse on itself, but the exotic matter at the center of Hawkins National Laboratory keeps pushing it outwards, resulting in a stable bridge.
Now, since exotic matter isn't really something that scientists have observed in reality, we don't know how it would behave when... say, being shot with Nancy's shotgun. But it wouldn't be too far-fetched to assume that destabilizing it would provoke great amounts of negative energy (since the matter has negative mass), and great amounts of this type of energy could literally melt everything on its path, explaining why the ceiling and floors suddenly turn into jello.
One big key to Dustin's plan after saving the 12 children from Vecna is to detonate a bomb right next to the exotic matter. That way, they would get rid of the only thing keeping the Upside Down alive and well. The wormhole would collapse on itself without the exotic matter, and the monsters from the Abyss wouldn't have a real means of coming back to Earth to terrorize us once again. The Stranger Things group doesn't even need to kill Vecna or the Mind Flayer; they just need to destroy the monsters' connection to our world to save the day.
Ultimately, exotic matter might be more important than it first appears in the world of Stranger Things. Without it, the Upside Down would've never come to be, and Will wouldn't have vanished one mysterious night in 1983. Now, it is the key to defeating Vecna once and for all. Let's just hope that every member of the group makes it out of the Upside Down first before closing it once and for all.