QCD axion mass -- dark matter density relation

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 the 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.