  We show that a Bose-Einstein condensate consisting of dark excitons forms in
GaAs coupled quantum wells at low temperatures. We find that the condensate
extends over hundreds of micrometers, well beyond the optical excitation
region, and is limited only by the boundaries of the mesa. We show that the
condensate density is determined by spin flipping collisions among the
excitons, which convert dark excitons into bright ones. The suppression of this
process at low temperature yields a density buildup, manifested as a
temperature-dependent blueshift of the exciton emission line. Measurements
under in-plane magnetic field allows us to preferentially modify the bright
excitons density, and determine their role in the system dynamics. We find that
their interaction with the condensate leads to its depletion. We present a
simple rate equations model, which well-reproduces the observed temperature,
power, and magnetic field dependence of the exciton density.
