in this galaxy here (with an arrow pointing to where the the galaxy used to be) everyone thought that their planet was flat.

A Rick Roll.

They should put a rick roll as a post credits scene of a movie. The closest I've seen to a troll post-credits scene is in Spiderman Homecoming where the post-credits scene is Captain America doing a PSA about patience, basically making fun of the audience for waiting through the credits fir nothing

A black card : They did their best, they weren’t very good at it.

The next Big Bang.

zoom out to the universe that created and contains this one, a fraction of a second has passed for them, this universe/the big bang is just what happens when you work out how to break the speed of light.

so long and thanks for all the fish

Yahweh does have a sense of humour.