  The large surface-to-volume ratio in atomically thin 2D materials allows to
efficiently tune their properties through modifications of their environment.
Artificial stacking of two monolayers into a bilayer leads to an overlap of
layer-localized wave functions giving rise to a twist angle-dependent
hybridization of excitonic states. In this joint theory-experiment study, we
demonstrate the impact of interlayer hybridization on bright and momentum-dark
excitons in twisted WSe$_2$ bilayers. In particular, we show that the strong
hybridization of electrons at the $\Lambda$ point leads to a drastic redshift
of the momentum-dark K-$\Lambda$ exciton, accompanied by the emergence of flat
moir\'e exciton bands at small twist angles. We directly compare theoretically
predicted and experimentally measured optical spectra allowing us to identify
photoluminescence signals stemming from phonon-assisted recombination of
layer-hybridized dark excitons. Moreover, we predict the emergence of
additional spectral features resulting from the moir\'e potential of the
twisted bilayer lattice.
