What Is Science's Most Intriguing Unanswered Question?

Part of the appeal of science is finding the answers to some of our most long-standing questions, but just as motivating are the questions it hasn't yet answered. Today we want to know what is the most fascinating unanswered question in science.

Tell us in the comments what single unanswered question from any scientific field — whether biology, astronomy, physics, robotics, or something else entirely — is most intriguing to you.

Image: Artist's conception of an exoplanet orbiting a star. / ESA, NASA, M. Kornmesser (ESA/Hubble), and STScI.