What Scientific Question Most Piques Your Curiosity?

Science is built upon questions, ranging from the deceptively mundane to the mind-bendingly complex. Today we want to know about the question that is most intriguing to you.

It could be a question that doesn't have an answer yet, or it could be a question that is all the more intriguing for having a partial answer, or it could even be a question with multiple answers. Whatever it is tell us about it in the comments — and what it is about it that fascinates you.

Image: ESA, NASA, and L. Calcada.