What is something about your world that is objectively true, but absolutely nobody living in it has any way of discovering?

I find this aspect of worldbuilding particularly fascinating because not every truth needs to be discoverable by the people living in a fictional world. There could be fundamental facts about their universe's origins, natural laws, history, or even the nature of their own existence that no civilization, no matter how advanced, could ever uncover. The interesting part is how those hidden truths shape the world regardless of whether anyone ever learns they exist.