The study of time-reversal symmetry and its breaking has been enormously fruitful in fundamental physics. A remaining challenge is to understand why time-reversal symmetry holds in the strong interaction, even though QCD does not forbid its violation. Addressing this problem leads led us to suggest the existence of a new particle, the axion, with remarkable properties. Axions can also provide the "dark matter" that we need in cosmology. Detecting the cosmic axion background is challenging, but it does not seem hopeless, and several powerful efforts around the world are aimed at that goal. I will especially focus on progress in the ALPHA collaboration.
