The MCU's Multiverse Vs. Dimensions: Ms. Marvel Finally Clears Up The Confusion
Words hold great meaning. That's why they can also cause great confusion when they're not chosen carefully. For example parallel/alternate dimensions, universes, and timelines are all used relatively synonymously in common conversation. They're all terms for hypothetical, self-contained planes of existence that coexist with our own. But in fiction, each of these terms means something distinct.
These distinctions are especially important in the Marvel Cinematic Universe, although, it has gotten increasingly difficult to differentiate between them as the MCU has grown. Nearly every movie and television show from the House of Ideas has its own rules on the subject, but at the end of the day, these seem to be the facts: "The multiverse" is a catch-all term for all the universes and dimensions in the MCU; Dimensions exist within universes, on the same plane of existence; There are different universes that run parallel to each other, which also contain their own dimensions. These alternate universes share many similarities to the "prime" universe (or as they call it in "Loki," the Sacred Timeline).
With the multiverse talk ramping up in Phase Four, the architects of the MCU have tried their best to set the record straight to avoid major confusion. And so far, the best place to find an explanation is "Ms. Marvel."
Embiggen your perception of the universe
In the fourth episode of "Ms. Marvel," Kamala Khan gets a lesson on the mechanics of dimensions from the Red Daggers. They explain to her that the ClanDestines come from the Noor Dimension, which coexists with our world but cannot be seen by the naked eye. Bollywood star Farhan Aktar's character Waleed goes on to explain that there are many dimensions around us that we cannot see. Using his handy map, he shows Kamala that these dimensions are the opposite sides of the same coin, which is really the best explanation of the various dimensions that are tethered to the MCU. Among them are the Quantum Realm in the "Ant-Man" movies, the Dark Dimension in "Doctor Strange," and the Mirror Dimension in "Spider-Man: No Way Home," just to name a few. These places can be accessed by "flipping" our world over to reveal one of its many sides.
But what about realms? Within the MCU, "realm" is synonymous with "dimension," and the two can be used interchangeably. A good example of this is when Thor explains the World Tree to Dr. Jane Foster in "Thor." He draws the branches of the tree in her notes, including Asgard, Earth (or Midgard), and the rest of the Nine Realms recognized by his people's mythology.
In "Thor: Love and Thunder," Thor and Jane go on to visit even more realms, such as Omnipotence City, the Shadow Realm, and Valhalla. These are all places that they can travel to within their universe, just as the ClanDestines were able to travel through the Veil of Noor and into the land of Kamala's ancestors. It's almost like each dimension is a different planet, but to get there, you don't travel through space in the conventional sense.
Catching the right plane (of existence)
As for alternate universes, these exist with their own mystical dimensions attached to them as well. These are the different places that Doctor Strange, the Scarlet Witch, and America Chavez visit in "Doctor Strange in the Multiverse of Madness," that Loki, Sylvie, and the TVA end up in throughout "Loki," and that the Watcher protects in "What If?" These alternate universes are just like the prime universe with various changes. Loki as an alligator, Agent Peggy Carter becoming a Super Soldier instead of Steve Rogers, and pizza coming in ball form instead of slices are just a few of the different scenarios that play out in these parallel universes.
But to make things complicated again, we've seen people use the dimensions of the Sacred Timeline to travel to alternate universes. Typically, it's someone with great power like Wanda Maximoff after studying the Darkhold or former Sorcerer Supreme Dr. Stephen Strange. Wanda uses the Astral Plane (which is a dimension of Earth-616 AKA the Sacred Timeline AKA the main MCU) to dreamwalk into her Earth-838 counterpart's body and take over. Strange does the same thing with Earth-838's Astral Plane to project himself into the deceased Strange variant buried on Earth-616. And in the post-credits scene of Sam Raimi's film, it's implied that Charlize Theron's Clea uses the Dark Dimension to travel the multiverse and uncover incursions.
Honestly, if you think about all of this stuff too much you're bound to get a headache. But dimensions, universes, and timelines don't seem to be going anywhere any time soon when it comes to the MCU. Thankfully, "Ms. Marvel" (and hopefully whatever nonsense I said here) was able to make things a little more digestible. At least, for now.