The latest clip from Spider-Man: Far From Home has revealed why there isn't a Spider-Man in Mysterio's dimension. Although Mysterio is best known in the comics as a con artist, trailers and TV spots for Spider-Man: Far From Home have consistently portrayed him as being far more than just that. He claims that the events of Avengers: Endgame tore a hole in the fabric of space-time, and that he fell through it from another dimension.
The jury's out as to whether or not to believe Mysterio, of course, but a clip recently shared on Ellen has fleshed out his claims a little, with Mysterio dubbing the MCU "Earth-616" and claiming to originate from Earth-833. What's more, if Mysterio is really being honest, he seems to be quite familiar with dimension-jumping; he even notes that Earth-616 and Earth-833 "share identical physical constants, Level 4 symmetry." One key difference, though, appears to be that his reality doesn't have a Spider-Man, which comes via a simple throwaway reference to Earth-833.
In the comics, Earth-833 was introduced in Dan Slott's "Spider-Verse" event. In this reality, it wasn't Peter Parker who was bitten by a radioactive spider; it was William Braddock. (Braddock was even referenced in Avengers: Endgame.) Rather than become Spider-Man, he was recruited into the multidimensional Captain Britain Corps, taking up the codename "Spider-UK." Braddock became a key player in "Spider-Verse," more used to jumping from one reality to another than the other versions of Spider-Man. Unfortunately, Spider-UK's story ended in tragedy. When he attempted to return home, Spider-UK learned that his entire dimension had been destroyed in a cosmic event known as an Incursion. It was probably restored later by the Fantastic Four, but Spider-UK never returned, instead operating as an interdimensional troubleshooter alongside other alt-reality Spider-Men.
Marvel didn't choose the designation "Earth-833" at random; they knew full well they were deliberately referencing an Earth that, in the comics, doesn't have a Spider-Man of its own. This fits perfectly with Mysterio's story, and comic book readers may well find him a little more believable as a result. Of course, the reality is that Marvel Studios is enjoying teasing viewers, with audiences divided over whether or not Mysterio is telling the truth. On the one hand, Mysterio is traditionally a super-villain and a serial liar, and played the superhero in his comic book debut; on the other, he does have precedent for traveling between the dimensions in the comics, and here he's referencing a reality that really wouldn't have a Spider-Man.
At the moment, it's almost impossible to say for certain - which is just the way Marvel and Sony want it to be. Spider-Man: Far From Home's marketing is clearly going to enjoy teasing viewers with Mysterio's claims, inviting everyone to pick a side ahead of the film's theatrical release.
