  Organic microcavities can be engineered to reach exotic quantum regimes of
strong and ultrastrong light-matter coupling. However, the microscopic
interpretation of their spectroscopic signals can be challenging due to the
competition between coherent and dissipative processes involving electrons,
vibrations and cavity photons. We develop here a theoretical framework based on
the Holstein-Tavis-Cummings model and a Markovian treatment of dissipation to
account for previously unexplained spectroscopic features of organic
microcavities consistently. We identify conditions for the formation of dark
vibronic polaritons, a new class of light-matter excitations that are not
visible in absorption but lead to strong photoluminescence lines. We show that
photon leakage from dark vibronic polaritons can be responsible for enhancing
photoluminescence at the lower polariton frequency, and also explain the
apparent breakdown of reciprocity between absorption and emission in the
vicinity of the bare molecular transition frequency. Successful comparison with
experimental data demonstrates the applicability of our theory.
