  Asymmetric dark matter theories generically allow for mass terms that lead to
particle-antiparticle mixing. Over the age of the Universe, dark matter can
thus oscillate from a purely asymmetric configuration into a symmetric mix of
particles and antiparticles, allowing for pair-annihilation processes.
Additionally, requiring efficient depletion of the primordial thermal
(symmetric) component generically entails large annihilation rates. We show
that unless some symmetry completely forbids dark matter particle-antiparticle
mixing, asymmetric dark matter is effectively ruled out for a large range of
masses, for almost any oscillation time-scale shorter than the age of the
Universe.
