WARNING: Spoilers for Ant-Man and the Wasp.

Just where has Captain Marvel been all these years? Next year will introduce viewers to Brie Larson's Carol Danvers, better known as Captain Marvel. Marvel's Kevin Feige has described Carol as "by far the strongest character we've ever had." Incredibly, Larson herself has made a throwaway comment that "she can move planets!" It's true that Larson may have been exaggerating in the heat of the moment, but the clear implication is that Captain Marvel is an absolute powerhouse.

That makes the mystery of Captain Marvel's absence in the MCU up till now all the more intriguing. Captain Marvel is set in the '90s, meaning this powerful hero has existed in the MCU for literally decades. So why hasn't she been mentioned before? Given the sheer number of super-soldier projects we've seen in the MCU, some sort of reference would have been expected. And why didn't Nick Fury call Captain Marvel in when the Chitauri invaded, or when Ultron threatened to render the human race extinct?

Related: 1990s Movies That Captain Marvel Must Reference

Avengers: Infinity War ended with Nick Fury sending out a sort of "cosmic S.O.S.," using an alien-looking pager. It was an amusing touch, a blend of alien technology and '90s science. But it suggested that Fury has always had the ability to call Carol Danvers for help, and yet never chose to use it. He didn't even summon Carol Danvers when he was being hunted down by Hydra, and had no idea who to trust. So where has Captain Marvel been all these years?

The Most Popular Theory: Captain Marvel has Been in Space

The most common theory is that Captain Marvel will head into space at the end of her solo film, and that Fury's pager sends a summons to her: come back home, we need you. On the face of it, that's a reasonable explanation. In one of his first statements about Captain Marvel, Feige confirmed that "much of the movie takes place in outer space, as you might think a Captain Marvel movie would." Thus, the argument goes that Captain Marvel will see the hero leave Earth for decades, perhaps fighting to bring an end to the Kree's many wars. It's even possible we've already seen evidence of Carol's presence, and that she instigated the truce between the Kree and the Xandarians in Guardians of the Galaxy.

Read More: Captain Marvel: The Kree-Skrull War Explained

By this argument, the reason Fury never used the pager before is that he believed it would take Captain Marvel a while to get back to Earth. By Avengers: Infinity War, though, he's lost all his other options; the Avengers are disbanded, Iron Man is missing, and now he's crumbling to dust. He sends out that mayday as a last, desperate Hail Mary. This also fits with one obscure hint from Kevin Feige. Asked where Captain Marvel has been, he gave a typically cryptic response. “The key is...‘out there,'" Feige responded, and most have assumed he was referencing outer space.

At the same time, though, this would be a major change in humanity's role in the cosmic side of the MCU. Back in 2014, James Gunn explained why he didn't want to introduce the character of Nova into the MCU at that stage. "I don't want Nova right now," he observed, "because I think Quill being the only earthling is important. That serves the entire movie-going audience and not just the handful of Nova fans." He was right, and the same point extends to Captain Marvel. In the Guardians of the Galaxy films, it's important that humans are initially treated with disdain, a backwater race whose sole cosmic representative is a down-on-his-luck sneak thief who shocks the galaxy when he wields an Infinity Stone. Captain Marvel's presence in space, let alone on the front lines of intergalactic wars and negotiating truces between alien empires, really does change that.

Page 2 of 2: Where We Think Captain Marvel Is

Another Possibility: Captain Marvel Is In The Quantum Realm

Another possibility, though, is that Captain Marvel has actually been in the Quantum Realm all this time. The Quantum Realm was introduced in 2015's Ant-Man as "a reality where all concepts of time and space become irrelevant." It's now been explored in detail by Ant-Man & the Wasp, although the pseudo-science behind the Quantum Realm is still a little murky. As Kevin Feige pointed out, dialogue in Ant-Man & the Wasp establishes that there are different "levels" to this plane of existence; Janet asks to be rescued from "the Wastelands beyond the Void." Presumably, it works like a sort of series of concentric circles: the deeper you get into the Quantum Realm, the more the laws of reality - laws like time and gravity - cease to apply. According to Ant-Man & the Wasp's mid-credits scene, the Quantum Realm contains mysterious phenomena known as "time vortexes." It seems likely these can be used to travel in time.

Crucially, we already know that the Quantum Realm will form some sort of narrative throughline from Ant-Man & the Wasp to Captain Marvel. Back in 2017, Dr. Spiros Michalakis - a quantum physicist who acts as a consultant for Marvel - explained why he felt the Quantum Realm was an interesting concept. "This is exciting for the future," he observed. "There are different ways that some of these ideas appear on-screen in a few years. Not just for Ant-Man, but also for Captain Marvel and all of the Marvel Cinematic Universe." So it seems the Quantum Realm will crop up in Captain Marvel, and most theories have it being the method for Avengers 4's time travel.

Related: There's No Way Avengers 4's Time Travel Will Make Sense

Back in 2016, Captain Marvel co-writer Nicole Perlman revealed that the character's origin story is being switched up a little because Marvel felt Carol's origin is "very similar to Green Lantern." We don't know exactly what changes have been made, but that does combine with the comments about Captain Marvel's power levels. It's possible that her MCU origin will actually be tied to the Quantum Realm; that Captain Marvel will somehow tap into quantum energy. Appropriately enough, Captain Marvel is strongly connected to an alien race known as the Kree, who - in the comics - are known for experimenting with quantum energy. They even created powerful artifacts known as Nega-Bands, inspired by the Quantum Bands worn by the superhero Quasar, and capable of tapping into the energies of the comic books' Quantum Zone.

If Captain Marvel is indeed strongly connected to the Quantum Realm, is it possible that's where she's been all these years? This would explain why Brie Larson is unlikely to age between her solo film and Avengers 4 for sure. Feige suggested Janet Van Dyne only aged because she didn't go into the heart of the Quantum Realm, where all the laws of reality break down. But if Captain Marvel had gone deeper than that, she'd age at a slower rate - and, perhaps, not even age at all. Meanwhile, it's worth noting that Ant-Man & the Wasp proved signals could be transmitted into the Quantum Realm. Thus Nick Fury's Mayday summons would still reach her.

-

Right now, the evidence is inconclusive. Not only will Captain Marvel explore the depths of space, the film is also stated to return to the Quantum Realm. The idea of Carol actually being inside the Quantum Realm does make sense, though, as it means Peter Quill was still the only human out in space at the time of Guardians of the Galaxy. It also fits with the fact that the Quantum Realm seems to be serving as connective tissue between the next few Marvel movies.

More: Captain Marvel May Be The Key To Ant-Man & The Wasp's Ending