  We performed spin-, time- and angle-resolved extreme ultraviolet
photoemission spectroscopy (STARPES) of excitons prepared by photoexcitation of
inversion-symmetric 2H-WSe$_2$ with circularly polarized light. The very short
probing depth of XUV photoemission permits selective measurement of
photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct
measurement of hidden spin polarization of bright and momentum-forbidden dark
excitons. Our results reveal efficient chiroptical control of bright excitons'
hidden spin polarization. Following optical photoexcitation, intervalley
scattering between nonequivalent K-K' valleys leads to a decay of bright
excitons' hidden spin polarization. Conversely, the ultrafast formation of
momentum-forbidden dark excitons acts as a local spin polarization reservoir,
which could be used for spin injection in van der Waals heterostructures
involving multilayer transition metal dichalcogenides.
