  The role and impact of spatial heterogeneity in two-dimensional quantum
materials represents one of the major research quests regarding the future
application of these materials in optoelectronics and quantum information
science. In the case of transition-metal dichalcogenide heterostructures, in
particular, direct access to heterogeneities in the dark-exciton landscape with
nanometer spatial and ultrafast time resolution is highly desired, but remains
largely elusive. Here, we introduce ultrafast dark field momentum microscopy to
spatio-temporally resolve dark exciton formation dynamics in a twisted
WSe$_2$/MoS$_2$ heterostructure with 55 femtosecond time- and 500~nm spatial
resolution. This allows us to directly map spatial heterogeneity in the
electronic and excitonic structure, and to correlate these with the dark
exciton formation and relaxation dynamics. The benefits of simultaneous
ultrafast nanoscale dark-field momentum microscopy and spectroscopy is
groundbreaking for the present study, and opens the door to new types of
experiments with unprecedented spectroscopic and spatiotemporal capabilities.
