So, I'd like to preface this by saying I dont have alot of movie or comic book fact to back up this theory, but i think it's really fun and something that could be explored.

The theory is about the Multiverse and how The Blip plays a factor into it, a huge factor actually. So, from what I've noticed watching everything post-blip is that it's never really stated that the people that were blipped actually died. They either vanished or disappeared, but I've not once heard them say that they died. Now that could just be everyone being polite and trying to use a better phrase for what happened OR it could be because those blipped people never actually died.

This takes me to my theory about the multiverse. We all know Dr. Strange's next movie is going to explore the multiverse, just based on the title alone. We also know that Spiderman and Scarlet Witch are going play a big part in that movie and all three of those characters were Blipped. My theory is that when the Blip happened, the people blipped went to another universe. Correct me if I'm wrong, but I believe Thanos always said he wanted to rid THIS universe of half its people. Now when he did that one the smaller planets he legit killed half the people, but with the stones they all disappeared into dust. What if, them fading into dust ripped them into a different universe.

No one seems to remember what happened to them while they were blipped and even Ant-Man doesnt know what happened when he gets back from the Quantum Realm, which is almost akin to another universe. As we all know, Dr. Strange, being the sorcerer supreme, tends to have more knowledge than the average bear, and definitely has the ability to research it all. He also is always looking for anomalies in the world. So what if when the vanished people were returned, they also brought with them some people that weren't originally here.

In Endgame, the people that remain always are talking about bringing everyone back. What if they mean just that.. instead of back from the dead, they brought them back from another universe. However, when Hulk snapped his fingers, he brought people with him that weren't here originally or maybe they were and we just didnt know it... but these new people that come back aren't really discovered because so many people came back at once that they slipped under the radar. I mean we all saw how crazy that hospital was in Wandavision when Monica came back. So, now we have these people from the multiverse that are here and they are staying under the radar trying to figure out their surroundings and it's not discovered until one of the bad guys from the universe makes a move and Dr. Strange picks up on it... those taking us into Multiverse of Madness and Spiderman 3

I know this theory could prob be busted open by people that have way more knowledge of the comic books and movies than me, but when u have covid and have to stay quarantined for 2 weeks, your mind starts to wander and you end up with this theory.