  We explore the viability of a boson dark matter candidate with an asymmetry
between the number densities of particles and antiparticles. A simple thermal
field theory analysis confirms that, under certain general conditions, this
component would develop a Bose-Einstein condensate in the early universe that,
for appropriate model parameters, could survive the ensuing cosmological
evolution until now. The condensation of a dark matter component in equilibrium
with the thermal plasma is a relativistic process, hence the amount of matter
dictated by the charge asymmetry is complemented by a hot relic density frozen
out at the time of decoupling. Contrary to the case of ordinary WIMPs, dark
matter particles in a condensate must be lighter than a few tens of eV so that
the density from thermal relics is not too large. Big-Bang nucleosynthesis
constrains the temperature of decoupling to the scale of the QCD phase
transition or above. This requires large dark matter-to-photon ratios and very
weak interactions with standard model particles.
