So this theory in short is a suggestion that the heroes in Infinity War didn't actually die. Which we all want to believe of course. 

The theory goes that the soul stone holds everyone that Thanos dusted away. That means Doctor Strange, Spiderman, Black Panther, Scarlet Witch, Bucky, Sam, and others may be trapped in the soul stone. 

In the comics, the soul stone takes people's souls and has a little world inside of it. In Infinity War it doesn't really show what powers the soul stone has. 

There is actually a lot of evidence for this. First of all, In the end of Infinity war we saw Thanos with Gamora in the soul stone...well a younger version of her to be exact*

Second, the soul stone in the past was known for making pocket dimensions for housing souls. 

With the evidence we have here. There is a possible chance that the dusted heroes are stuck in the stone. 

*As for Gamora being in the soul stone. There's a lot of evidence pointing towards her soul being stuck in the stone. 

So far Gamora's the only one who really has confirmed status on where she is. The directors confirmed it by saying and I quote:

"She in fact is, yes. It was an attempt on our part – because we don't like two dimensional roles or three dimensional villains every villain is a hero in their own story and as insane and psychotic and brutal and violent as Thanos is he's a more complex villain if you go on a journey with him emotionally. He does care for things and it is complicated for him to execute his plan and it cost him something. He said at the end that it cost him everything and that it was the only thing he loved which was Gamora which is why we put him back with her at the end. I just want to reiterate with the audience that he does feel true emotion even though he is a monster."

Then a few days later they clarified the confirmation saying, "The power that it took Thanos to click his fingers, he's so dramatic that if you look closely at the film and the damage of the gauntlet," Joe Russo said. "You look closely at the very last scene, it damages his arm dramatically. And that moment when he clicks his fingers sends him into this ethereal plane inside of the Soul Stone where as part of his quote unquote hero's journey in the film, he has one last moment to convey a sense of guilt about what he's done. It illustrates what a complicated character he is. So, to clarify, it's a spiritual representation of her and obviously that's something that the Soul Stone has power to do. But really it's there to illustrate his pain and his guilt. I wouldn't read much more into it beyond that."

They also said that the Gamora in the soul stone was a spiritual representation of her. 

>.< OUCH, they're really committed to having us believe that everyone's dead. I mean its sad to hear that they're saying that Gamora's dead dead. Which is very painful to hear and tell. Guessing from how they originally answered yes, this may or may not be accurate. Only time will tell if Gamora and all the heroes come back or stay dead. 

yikes that was pretty cynical, let me end this on a happy note

There much better

*Let me know if I missed any information on this theory!

have a nice night^^