  The possible phases and the nano-scale particle correlations of
two-dimensional interacting dipolar particles is a long-sought problem in
many-body physics. Here we observe a spontaneous condensation of trapped
two-dimensional dipolar excitons with internal spin degrees of freedom from an
interacting gas into a high density, closely packed liquid state made mostly of
dark dipoles. Another phase transition, into a bright, highly repulsive plasma
is observed at even higher excitation powers. The dark liquid state is formed
below a critical temperature $T_c \approx 4.8K$, and it is manifested by a
clear spontaneous spatial condensation to a smaller and denser cloud,
suggesting an attractive part to the interaction which goes beyond the purely
repulsive dipole-dipole forces. Contributions from quantum mechanical
fluctuations are expected to be significant in this strongly correlated, long
living dark liquid. This is a new example of a two-dimensional atomic-like
interacting dipolar quantum liquid, but where the coupling of light to its
internal spin degrees of freedom plays a crucial role in the dynamical
formation and the nature of resulting ground state.
