  The non elementary-boson nature of excitons controls Bose-Einstein
condensation in semiconductors. Composite excitons interact predominantly
through Pauli exclusion; this produces dramatic couplings between bright and
dark states. In microcavities, where bright excitons and photons form
polaritons, they force the condensate to be linearly polarized--as observed. In
bulk, they also force linear polarization, but of dark states, due to interband
Coulomb scatterings. To evidence this dark condensate, we thus need indirect
processes, like the shift it induces on the (bright) exciton line.
