Abstract: The hypothetical QCD axion would explain why the strong interactions obey Time-reversal symmetry, and it could also make up the Dark Matter of the Universe. I argue that, within reasonable assumptions, it should be possible to predict the axion's mass if it makes up all of dark matter. I discuss the somewhat intricate physics involved in making this prediction. I predict an axion mass of 26 +- 3 micro-electron volts.
This colloquium is part of the "Landelijk Seminarium @ Nikhef/CWI, Z011"