When a massive star dies, a supernova explosion occurs, so powerful that a single flash outshines an entire galaxy. In this fire, billions of axions are born — invisible particles that leave almost no trace. Every second, trillions of these tiny things fly right through each of us, completely unnoticed. These invisibles are thought to be dark matter — the unseen substance that keeps galaxies from flying apart.
But occasionally, an axion bumps into a magnetic field, and for a fleeting moment, the invisible becomes visible: it turns into a gamma-ray photon, a packet of light. For the first time, astrophysicists summed up such "flashes" from all supernovae across the history of the Universe and detected a faint diffuse glow. This allowed them to precisely measure the interaction strength of axions, and perhaps soon we'll learn what cosmic darkness is made of. These data set tight constraints on theories and inspire the construction of new gamma-ray telescopes.
The name 'axion' comes from a brand of laundry detergent: the particle was dubbed so because it was meant to 'wash away' a theoretical problem.