SPOILER WARNING: This article contains major spoilers for Avengers: Infinity War, in theaters now.
Make no mistake: Captain Marvel is well on her way to the Marvel Cinematic Universe. Ahead of her solo movie, which arrives early next year, Nick Fury put out the call to her in Avengers: Infinity War after Thanos killed half of all life in the universe. However, since Captain Marvel is set in the 90s and she has been noticeably absent since then, that begs an important question: Where has she been all this time? We think we may know the answer: out in the multiverse.
First, let's go over that Avengers: Infinity War post-credits scene. As we now know, Thanos the Mad Titan was successful in uniting the Infinity Stones, which made him the most powerful being in the universe. This enabled him to carry out his one true goal of killing half of all life in the universe, in an insane effort to "save" the universe from using up its finite resources.
So, in the post-credits scene, Nick Fury and his right-hand woman Maria Hill began to feel the affects of that. As he witnessed several people turn to dust, including Hill, he immediately went for his bag. He pulled out a strange device that looked like a modified pager. As he himself began to disintegrate, he sent a signal. When it finally connected, it revealed Captain Marvel's logo, heralding her arrival in both her solo film and the as-yet untitled Avengers 4.
Since then, many fans have speculated that Carol Danvers, aka Captain Marvel, has been in space all this time -- and for good reason. The character is intrinsically linked to the Kree, an alien species introduced to the MCU in 2014's Guardians of the Galaxy. In the comics and likely in the film, Carol received her powers when her DNA was meshed with that of the Kree soldier Mar-Vell. Further, the movie is confirmed to tackle The Kree-Skrull War, which will find Carol "caught in the middle of a galactic war between two alien races."
It's entirely feasible, then, that Carol has spent the past two decades in outer space. However, that doesn't quite explain why she will stay so young between the next few MCU films. That is to say, Brie Larson will fill the role for both Captain Marvel and Avengers 4, but we don't know how or why she hasn't aged between those two films. If she spent that time in the multiverse, though, that could answer this critical question.