What's outside the visible world

In ancient times, people stand on the ground and asked a question:"how wide is the ground?" They never got an explicit concept until they climbed high enough in the third dimension.
Now people wander in the cosmos and ask a similar question:"how big is cosmos?"...
Some information can be found in astronomical materials/reports that the visible world is about **billion light years. Don't mind the exact number. The question here is that:" what's outside the visible world?"
A. The visible world is infinite big. This answer is somewhat alike some ancient people's concept about the ground.
B. Nothing. This answer is a bit fishy. It render people wonder if it is an answer.
C. Invisible worlds.
D. No one knows.
Declaration: This is a game of philosophy here.
Thank you.