  Resolving the momentum degree of freedom of excitons - electron-hole pairs
bound by the Coulomb attraction in a photoexcited semiconductor, has remained a
largely elusive goal for decades. In atomically thin semiconductors, such a
capability could probe the momentum forbidden dark excitons, which critically
impact proposed opto-electronic technologies, but are not directly accessible
via optical techniques. Here, we probe the momentum-state of excitons in a WSe2
monolayer by photoemitting their constituent electrons, and resolving them in
time, momentum and energy. We obtain a direct visual of the momentum forbidden
dark excitons, and study their properties, including their near-degeneracy with
bright excitons and their formation pathways in the energy-momentum landscape.
These dark excitons dominate the excited state distribution - a surprising
finding that highlights their importance in atomically thin semiconductors.
