Warning: SPOILERS for Doctor Strange in the Multiverse of Madness.
Doctor Strange in the Multiverse of Madness explores the concept of the multiverse, and may have established Earth-838 as the MCU's version of the Ultimate universe from the comic books. The multiverse first appears in Loki and Spider-Man: No Way Home, establishing that multiple different universes exist within the MCU, which feature different versions of popular heroes and villains, such as Sylvie, a variant of Loki, and Tobey Maguire's and Andrew Garfield's Spider-Men. However, the universe that Multiverse of Madness spends most of its time in may hint at a cinematic version of the Ultimate universe.
In Doctor Strange in the Multiverse of Madness, Earth-838 is introduced as the home of the Illuminati, a group of heroes made up of Captain Carter, Black Bolt, Captain Marvel (Maria Rambeau), Mr. Fantastic, Professor X, Karl Mordo, and formerly Stephen Strange. The universe seems to have progressed further into the future than the main universe of the MCU, confusingly named Earth-616, the same designation as the Marvel comics universe (the MCU had previously been designated Earth 1-99999 prior to the movie's release). As America Chavez and the MCU's Doctor Strange explore the universe, they find the Earth-838 version of New York to be advanced, with alternate road crossings and memory machines on every corner.
Related: Vision’s Absence Is Doctor Strange 2’s Greatest Sin
Earth-838 seems to have a similar history to Earth-616, although its heroes appear slightly different, such as Captain Carter becoming the first Avenger instead of Captain America, just like in What If?. However, the Illuminati (presumably this universe's primary team of heroes), are
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