  We investigate, in the context of asymmetric dark matter (DM), a new
mechanism of spontaneous co-genesis of linked DM and baryon asymmetries,
explaining the observed relation between the baryon and DM densities,
Omega_DM/Omega_B ~ 5. The co-genesis mechanism requires a light scalar field,
phi, with mass below 5 eV which couples derivatively to DM, much like a `dark
axion'. The field phi can itself provide a final state into which the residual
symmetric DM component can annihilate away.
