There are two ways to start a story about the axion. One is to explain that this hypothetical particle could be the key to a major problem in the Standard Model of particle physics, which describes all of the known fundamental particles. The beauty of the axion is that there is a second, equally significant beginning to its story. We now suspect the axion may also be the answer to one of the most important questions—if not the most important question—in all of particle physics and astronomy: What is dark matter? Dark matter is the term that researchers use for the invisible substance that seems to dominate the formation of cosmic structure and to make up the majority of all tangible matter in the universe. The first thing I like to tell people about dark matter, to help them develop some intuition for this strange idea, is that dark matter is a terrible name for this stuff, whatever it is. The term is often attributed to Swiss astrophysicist Fritz Zwicky, who proposed the existence of