This was a story read on freesfonline at some point... A group of beings exists in the hollowed center of a planet, the space being their known Universe.
Generations go on, with technological developments always negotiating the truism that, for this society, to dig is to create a pile. That's not the exact phrase, but the author really thoughtfully explores this concept: the universe is what's inside the unknown, 'making' space necessitates taking it up. So explorers are generally discouraged, as they create annoying rubble for the society.
But the story charts efforts to discover what lies beyond the known Universe, in the dense, rock layers surrounding their world. I think they make it to the surface, after generations of cautious attempts.