  The concordance Lambda Cold Dark Matter (Lambda-CDM) model for the formation
of structure in the Universe, while remarkably successful at describing
observations of structure on large scales, continues to be challenged by
observations on galactic scales. Fortunately, CDM models and their various
proposed alternatives make a rich variety of testable predictions that make the
Local Group and our own Milky Way Galaxy key laboratories for exploring dark
matter (DM) in this regime. However, some of the most definitive tests of local
DM require microarcsecond astrometry of faint sources, an astrometric regime
that is a unique niche of SIM Lite. This chapter explores the important and
distinct contributions that can be made by SIM Lite in the exploration of
galaxy dynamics and DM on galaxy scales and that have cosmological
consequences. Key areas of potential SIM Lite exploration include (1) measuring
the shape, orientation, density law, and lumpiness of the dark halo of the
Milky Way and other nearby galaxies, (2) determining the orbits of Galactic
satellites, which may be representatives of late infall from the hierarchical
formation of the Milky Way, (3) ascertaining the distribution of angular
momentum and orbital anisotropy of stars and globular clusters to the outer
reaches of the Galactic halo, dynamical properties that hold clues to the early
hierarchical formation of the Galaxy, (4) measuring the physical nature of DM
by placing strong constraints on the phase space density in the cores of nearby
dSph galaxies, and (5) reconstructing the dynamical history of the Local Group
through the determination of orbits and masses of its constituent galaxies.
