  High degree of valley polarization of optically active excitons in
transition-metal dichalcogenide monolayers (TMD-MLs) is vital in valley-based
photonic applications but known to be likely spoiled by the intrinsic
electron-hole exchange interactions. In this study, we present a theoretical
investigation of the valley and optical properties of finite-momentum dark
excitons in WSe$_2$-MLs by solving the density-functional-theory(DFT)-based
Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. %We
reveal that, in general, finite-momentum excitons are actually well immune from
the exchange-induced valley depolarization, except for those with specific
exciton momenta coincident with the $3\sigma_v$ and $3C_2'$ symmetries in the
$D_{3h}$ point group of TMD-MLs. We reveal that, unlike the bright exciton
inevitably subjected to electron-hole exchange interaction, inter-valley
finite-momentum dark excitons in WSe$_2$-MLs are well immune from the
exchange-induced valley depolarization and inherently highly valley-polarized
under the enforcement of the crystal symmetries. More importantly, the superior
valley polarizations of the inter-valley dark excitons in WSe$_2$-MLs are shown
almost fully transferable to the optical polarization in the phonon-assisted
photo-luminescences because of the native suppression of exchange-induced
depolarization in the second-order optical processes. The analysis of
phonon-assisted photo-luminescences accounts for the recently observed
brightness, high degree of optical polarization and long lifetime of the
inter-valley dark exciton states in tungsten-based TMD-MLs.
