It's actually kind of a relief that I have to keep explaining it, if it means other worlds don't even know what it IS.

Unfortunately I don't exactly know either. All I know is old books say that there used to be a might civilization that covered the entire world, and that the people could fly in the sky and walk on the moon. But then there was a big war that used nuclear bombs, which could level entire cities in one blow. The world we live in now is just the tiny sliver of people who survived, eking out a living among ruins.

They say nuclear bombs are so terrible they poison the very Earth, and that anyone who even GOES there will get radiation sickness and probably die.