Stop the dark sky at bottom of world

I have this dark sky thing that appears down low in the world (below y 61) that i'm trying to get rid of in my custom dimension because it looks dumb. I have a feeling I can't get rid of it but I'd at least like it to only show up below terrain that the players might be on (generation ends about y = 0).
So far I've tried moving the world up to be above y=61 but the black sky just comes up with it. I also tried extending the bottom of the world down but that had no effect. Does anyone know where this comes from and what are the rules for it appearing?
(I've also noticed that the transition between light and dark is instant in my dimension however when I moved the world up it faded in, weird...)