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 show why making this prediction is difficult in practice, and how to overcome these difficulties. I predict an axion mass of 26 +- 3 micro-electron volts.