  Monolayers of semiconducting transition metal dichalcogenides (TMDCs) with
unique spin-valley contrasting properties and remarkably strong excitonic
effects continue to be a subject of intense research interests. These model 2D
semiconductors feature two fundamental intravalley excitons species - optically
accessible ' bright' excitons with anti-parallel spins and optically inactive
'dark' excitons with parallel spins. For applications exploiting radiative
recombination of bright excitons or long lifetime dark excitons, it is
essential to understand the radiative character of the exciton ground state and
establish the energy separation between the lowest energy bright and dark
excitons. Here, we report a direct spectroscopic measure of dark excitons in
monolayer MoSe$_2$ encapsulated in hexagonal boron nitride. By applying strong
in-plane magnetic field, we induce mixing and splitting of bright and dark
exciton branches, which enables an accurate spectroscopic determination of
their energies. We confirm the bright character of the exciton ground state
separated by a 1.5~meV gap from the higher energy dark exciton state, much
smaller compared to the previous theoretical expectations. These findings
provide critical information for further improvement of the accurate
theoretical description of TMDCs electronic structure.
