Avengers 4 speculation was common even before Avengers: Infinity War's release, but ever since Ant-Man and the Wasp came out, there have been numerous Marvel fans who have been theorizing about how the Quantum Realm will play into the Phase 3 capper. One particularly intriguing theory that's made the rounds is that the people who Thanos wiped from existence aren't actually dead. Instead, their souls were transported to the Quantum Realm, and one Marvel fan believes that Thor: The Dark World gives credence to this idea.
Thor: The Dark World definitely isn't the most beloved of the Marvel Cinematic Universe movies, but it's nonetheless an important chapter of this franchise, mainly for introducing the Aether, a.k.a. the Reality Stone. However, during the sequel, when Jane Foster is taken to Asgard after being infected by the Aether, there's a scene where she's being studied in a quantum field generator, or as it's known among the Asgardians, a soul forge. You can refresh your memory of that moment by watching the clip below.
Redditor u/learinferno believes that this confirmation means that souls are connected to the Quantum Realm. This therefore strengthens the idea that the unlucky half of the universe that was turned to dust when Thanos snapped his Infinity Stones-powered hands didn't die, but were instead transported to the Quantum Realm. Some have also applied this souls theory to the Soul Stone, as Thanos saw a young Gamora within Soulworld after snapping his fingers, but the Quantum Realm also makes for an interesting option, too.
Just like the Dark Dimension seen in Doctor Strange, the laws of nature don't work in the Quantum Realm like they do in our plane of existence. You can't quantity or measure a soul here, but in the Quantum Realm, they have actual form, and that's where we'll find the people who disappeared at the end of Avengers: Infinity War. Of course, it's not like back when Thor: The Dark World was being made that Kevin Feige and the other MCU bigwigs knew that an Asgardian Soul Forge would be connected to the Quantum Realm. Then again, given how far ahead these movies are planned, maybe they did know.
Michael Douglas, a.k.a. the MCU's Hank Pym, has already confirmed that the Quantum Realm will be an important part of Avengers 4. The question is how. Presumably we'll catch a glimpse of it when Scott Lang breaks out of this dimension, having been trapped there in the Ant-Man and the Wasp mid-credits scene, but beyond that, Marvel is keeping us in the dark. That said, there is one other popular theory concerning the Quantum Realm that gathered a lot of traction.
Janet van Dyne told Scott Lang before he went back into the Quantum Realm not to get sucked into a time vortex. Assuming there's a long time gap between Avengers: Infinity War and Avengers 4, when Scott breaks free, years will have passed in our reality even though it only moments for him. Keeping that in mind, many believe that the surviving heroes will use the Quantum Realm to travel back in time and undo Thanos' genocide. If we assume that this idea is correct, then those who were dusted are indeed dead, but they'll be restored to life once the timeline has been changed.
We'll have all our questions answered when Avengers 4 is released on May 3, 2019. In the meantime, check out what else the MCU has in development by browsing through our Marvel movies guide.