  We discuss how astrophysical observations with the Maunakea Spectroscopic
Explorer (MSE), a high-multiplexity (about 4300 fibers), wide field-of-view
(1.5 square degree), large telescope aperture (11.25 m) facility, can probe the
particle nature of dark matter. MSE will conduct a suite of surveys that will
provide critical input for determinations of the mass function, phase-space
distribution, and internal density profiles of dark matter halos across all
mass scales. N-body and hydrodynamical simulations of cold, warm, fuzzy and
self-interacting dark matter suggest that non-trivial dynamics in the dark
sector could have left an imprint on structure formation. Analysed within these
frameworks, the extensive and unprecedented datasets produced by MSE will be
used to search for deviations away from cold and collisionless dark matter
model. MSE will provide an improved estimate of the local density of dark
matter, critical for direct detection experiments, and will improve estimates
of the J-factor for indirect searches through self-annihilation or decay into
Standard Model particles. MSE will determine the impact of low mass
substructures on the dynamics of Milky Way stellar streams in velocity space,
and will allow for estimates of the density profiles of the dark matter halos
of Milky Way dwarf galaxies using more than an order of magnitude more tracers.
In the low redshift Universe, MSE will provide critical redshifts to pin down
the luminosity functions of vast numbers of satellite systems, and MSE will be
an essential component of future strong lensing measurements to constrain the
halo mass function. Across nearly all mass scales, the improvements offered by
MSE, in comparison to other facilities, are such that the relevant analyses are
limited by systematics rather than statistics.
