I'm sorry I can't give any details, but I just remember I once saw a Japanese anime series.
I think it was about a (future) Earth where the Earth was suddenly surrounded by a kind of (blue) energy sphere in the sky. How they were looking up and realizing it was blocking everything so they couldn't leave the planet anymore, or something like that. It was more about the figures and how they reacted. No monsters or super powers etc involved. Probably not even a villain!
And it was a kind of mirror/alternate world or something like that "up there". I know that sounds like the Upside-Down movie from 2012 but I'm sure it was an anime (and most likely a series and not a movie) and should be slightly older than that. But it wasn't really old. I remember it looked already CGI-enhanced so I'd say early 2000s.
I remember a dark, sad mood like it was changing the world to the worse, not the better. However my memory is not a really depressing story. Neither was it a dark sci-fi like Final Fantasy or whatever, more of a "mystery" instead.
If I would have to guess I'd say the (English) title was something like Twin X or something abstract like that. Maybe containing "sky" or "mirror" in the name?
Since I don't remember much of the story sadly - only a mood - I think it can't be a long story. Maybe one season perhaps. And its painting style was the typical "anime" one. If I had to guess, I would say the "normal" 2D paint style with slightly modernized backgrounds, so no 3D-generated characters etc.