How Did X Become the Unknown (and so much else)?

Despite being one of the rarest letters in English, the letter X is everywhere. We sign letters with it, we rate movies with it, we name all sorts of things after it. We’re kind of obsessed with it. And that’s probably all thanks to math, where x stands for the unknown. But how x took on this role is a bit of an unknown itself, and the theories that are out there are truly bizarre.