OK, so if you saw Captain Marvel, then you know that Captain Marvel/Carol Danvers is not only a badass, but also the most powerful superhero in MCU and is going to play a major role in defeating Thanos.
In fact, we were clued into it last year when we saw Nick Fury page Carol in a post-credit scene in Avengers: Infinity War.
And in a post-credit scene in Captain Marvel, we see Carol responding to Nick's page by showing up to the New Avengers Facility looking for him.
But all these things lead to one big question: Where has Captain Marvel/Carol been these last 20-plus years?! Especially considering that Nick could've contacted her anytime and that the world has been under attack several times by alien lifeforms!
Well, in this just released clip from Avengers: Endgame, Carol, when asked, has a pretty good explanation to why she hasn't been around:
So there you have it: Captain Marvel/Carol has been kicking ass and single-handedly saving other planets across the universe for the last two decades. And it makes sense (especially seeing how they established her character in the movie) that she didn't feel a need to return if she knew the Avengers were on Earth to defend it.
Now I am only hoping Endgame will answer where Goose has been the last 20-plus years ('cause you know he is still alive!).
Checkout the entire Captain (here to motherfucking save the day) Marvel scene here: