The Spider-Man: Far From Home trailer has revealed that Mysterio is from an alternate universe to the main MCU. Prior to the release of the new look at the Spider-Man: Homecoming sequel, there was a lot of focus of what Avengers: Endgame spoilers it would reveal. Many suspected that would be the death of Tony Stark - which is definitely present - but it wasn't the biggest reveal.
When Peter Parker first meets Quentin Beck aka Mysterio in the Spider-Man: Far From Home trailer, the strange agent of Fury says "we could use someone like you on my world." As Nick Fury clarifies, "the snap tore a hole in our dimension", essentially confirming the MCU exists in a proper multiverse and that Spider-Man: Far From Home, previously only viewed as a European spy romp, will be the first movie to explore that.
That's a shocking reveal, but one that can make sense. The multiverse is a well-established aspect of Marvel Comics so, like cosmic gems and time travel, Spider-Man: Far From Home's Mysterio is just the latest step in things getting very weird in the MCU. While we've only seen on Earth so far, there are infinite other dimensions - some abstract like the Dark Dimension or Quantum Realm - but others variants of the prime Marvel timeline.
How Mysterio's Alternate Dimensions Was Created (Or Found)
From what Mysterio and Fury say, it seems like Beck's dimension always existed, but until the events of Avengers: Endgame it was isolated from our own, with the snap causing them to bridge.
There were actually four snaps across the events of Avengers: Infinity War and Endgame: Thanos' first to dust half of all life, his second to destroy the Infinity Stones, Hulk's to bring the dead back, and Iron Man's to wipe out 2014 Thanos' forces. Given how the latter two hero snaps were undoing mistakes, it's likely to blame falls on Thanos himself, possibly the second as the atomizing of the stones released unknown and unkept power.
What exactly destroying the Infinity Stones meant has been a key question since Endgame released. In Avengers: Endgame, the Ancient One stated that the Infinity Stones cause the flow of time and their removal could have grave consequences. There she was talking about it in reference to time travel, but something similar must surely hold true of their actual destruction; Thanos' actions before Endgame even began could have the biggest effect.
Why Has Mysterio Come To The Main MCU Universe?
But that's the method of how Mysterio was able to jump from Earth to Earth, the bigger question is why. Thankfully, this is actually the key to understanding what exactly's going on with Spider-Man: Far From Home's story.
Spider-Man: Far From Home sees Peter Parker called in to help Fury and his team take on Elementals, powerful creatures that appear to be loosely based around classic Spidey villains Molten Man, Hydro-Man and Sandman (and not the Elementals of the comics). Given how Mysterio seems to view them as his problem, it would seem that they are from his dimension or released by the effects of the snap, with him coming over to project all Earths as a sense of duty.
However, there's a wrinkle to all this: Mysterio is a villain, one of Spider-Man's most famous rogues yet to come to the big screen. So while he may present himself as working for Fury and forming a friendship with Peter Parker, there have to be ulterior motives at hand. It may be that, instead of working against the Elementals, they're part of some more nefarious scheme. Although going back to the comics, the truth may be something different altogether that actively works against the alternate dimension statement...
Is Mysterio Lying About Alternate Dimensions?
Mysterio's a master of deception. His very first appearance in Spider-Man comics saw special effects maestro Quentin Beck attempt to trick the populace into thinking he was a superhero by using advanced visual tricks, only to be foiled by Spider-Man. He's since become a figure who does jump from dimension to dimension one crime sprees, so there is comic precedent for this shock origin, but could the truth be murkier?
Considering that the MCU version of the web-slinger has been very much going back to the early comics for inspiration, it's distinctly possible that's what we're seeing here: taking advantage of the unkempt and unclear ramifications of Thanos' snaps, is a gifted illusionist attempting to make himself appear a hero by faking the attack of several extra-dimensional beings. Think Ozymandias from Watchmen meets Syndrome from The Incredibles, attempting to take advantage of the lack of available heroes to position himself as one. That would also fit with how Vulture in Homecoming reflected Spider-Man's own aspirational background.
That wouldn't disprove alternate dimensions in the MCU - Doctor Strange, Ant-Man and Avengers: Endgame already dealt with multiple to varying degrees, with the latter making at least a couple new divergent timelines - but, when dealing with Mysterio, it's always best to have your wits about you.
