Beware! There are more Captain Marvel spoilers here than we know what to do with.
Captain Marvel has two post-credits stingers: one featuring a distractingly massive reveal, and the second a seemingly innocuous comedy beat involving a new fan fave character. And while the latter has been widely written off as a humorous moment a la the Schwarma sequence from the end of Avengers, we actually think this final moment potentially explodes a huge piece of established Marvel canon and throws the future (and past) of the MCU into complete disarray.
There isn't much you need to know about the narrative of the film for the purpose of this theory other than the fact that at some point in 1995, Carol ends up in possession of the Cosmic Cube A.K.A. the Tesseract A.K.A. the Space Stone. Then her cat/alien companion Goose swallows it. Then, in a cute final stinger after the film's credits, the Flerken coughs it up in front of Nick Fury's conspicuously empty office chair.
The scene prompts questions. When does the moment take place? At first, it appears likely that it would be within the timeframe of the film itself. But when we look back at Nick Fury's comment to Agent Coulson that the Tesseract would eventually "show up," time has already passed and Nick is clearly happy to keep the whereabouts of the stone to himself and his cuddly Flerken.
We also have to ask how exactly the stone got into the possession of the Kree leader Mar-Vell (Annette Bening) who was using it when Carol came upon it. According to the timeline of the MCU, the last person to have the stone before Mar-Vell was Howard Stark... So how exactly did it exchange hands and why wouldn't the MCU have utilized one of their favorite tools—a Howard Stark flashback—to show this?
Canonically, there has yet to be an MCU stinger that is set in the past. The emptiness of Fury's chair feels important, reminiscent of the recent stingers we've seen of the world following the Snap. Taking that into consideration, this might mean Goose just puked up the Space Stone decades after its discovery, at the same time that it's lodged into the middle knuckle on Thanos' favorite accessory.
However, we have a different theory that goes back to the comics that birthed the MCU, one that would offer up a new way out for the characters facing the aftermath of Thanos' decimation.
Though in the MCU it's known as the Tesseract, or the Cube, the geometric vessel that held the Space Stone has a comic book background that is far more complex and, most importantly, not focused on one single cube. Yep, in the comics, multiple Cosmic Cubes exist. They're known as Cosmic Containment Units, which fits very well with the purpose of the stone as we know it in the movie, don't you think?
The concept of multiple cubes was first introduced in Jim Starlin, Ron Lim, Al Milgrom, Ian Laughlin, Christie Scheele, and Jack Morelli's The Infinity War. This comic saw Adam Warlock deal with the fallout of having taken on the Infinity Gauntlet in the previous miniseries named, well, The Infinity Gauntlet. In preparation for reaching omnipotence, Warlock expelled the good and evil sides of himself, unintentionally creating the Goddess and the Magus, the latter of which decides to fight Thanos and that to do so he'd need all eight Cosmic Cubes. In case you can't see what we're getting at, there's comic book canon that supports the idea of multiple cubes, and we think that Goose may have just puked one up while Thanos extracted the Space Stone from another.
This would obviously be huge for the sad Avengers who are lost in the wake of Thanos' power, and the original Cube could help them. Another supporting factor for this theory is that Mar-Vell would never have had to make a lightspeed engine if she had the Space Stone as we've seen that it gives its holder the ability to teleport. So could it be that this Tesseract is slightly less powerful? Whatever the answer, the truth is out there and we think that the future of the MCU may just rely on the sticky hairball-covered Tesseract laying in Nick Fury's post-Snap office.