Why does QCD seem to preserve CP? The QCD axion is an attractive dark matter candidate able to answer the former question. However, the invisible axion mechanisms do not predict its mass, so that in general there is no clue on where to look for this candidate experimentally. In this seminar we will propose the simplest realistic grand unified theory able to host the axion and predict its mass. In this context, we find an appealing theory that provides a dark matter candidate with a definite mass, a solution to the strong CP problem, neutrino masses, and unification of forces. As we will show, this theory can be tested in future axion and proton decay experiments.