Idk, I don't think most scifi pushes the envelope of what we can imagine, rather it provides a convenient escape to galaxies less incomprehensible than the bewilderment of earth where the author can make a point about spacewar and unstoppable mindless empires.

shrugs

Scifi (like any other genre) needs to continually reaffirm its association with creativity, not assume because paper thin character types are fighting spacewars for feudal empires and space corporations that it counts as pushing the envelope of our imaginations.

/end side rant

He bathes in the black sun

Plus, black holes may do contain universes. This year there was some evidence pointing that our universe is actually trapped inside one... ref: https://futurism.com/universe-trapped-inside-black-hole

They really drive home the point that we can't understand time