Void Century: how can a whole century go by without anyone knowing what happened? In One Piece, it seems like little to no people knows what happened during the Void Century.

How is this possible?
The World Government is heavily protective of whatever happened during the Void Century, and actively seeks to keep people in the dark about it. They burned every book they could find that contained info on the subject, and killed everyone they found who knew about it while preventing as much info from leaking as possible. The Ohara incident is a direct example of that; its inhabitants knew too much about the Void Century to be allowed to live.