  We predict that the collision of two fully dark exciton condensates produces
interference fringes which are not only dark but also bright. So, quite
surprisingly, the collision of coherent states made of dark excitons produces
light. This remarkable effect, which is many-body in essence, comes from the
composite boson nature of excitons, through the fermion exchanges they can have
which transform dark states into bright states. The possibility of optically
detecting quantum coherence in a regime where the system is hidden by its total
darkness, was up to now considered as hopeless.
