  The formation of a dense Bose-Einstein condensate in dark spin states of
two-dimensional dipolar excitons is shown to be driven by a dynamical
transition to the long-lived dark states. The condensate is stabilized by
strong dipole-dipole interactions up to densities high enough for a dark
quantum liquid to form. The persistence of dark condensation was observed in
recent experiments. A model describing the non-equilibrium dynamics of
externally driven coupled dark and bright condensates reproduces the step-like
dependence of the exciton density on the pump power or on temperature. This
unique condensate dynamics demonstrates the possibility of observing new
unexpected collective phenomena in coupled condensed Bose systems, where the
particle number is not conserved.
