  Cold Dark Matter (CDM) theory, a pillar of modern cosmology and astrophysics,
predicts the existence of a large number of starless dark matter halos
surrounding the Milky Way (MW). However, clear observational evidence of these
"dark" substructures remains elusive. Here, we present a detection method based
on the small, but detectable, velocity changes that an orbiting substructure
imposes on the stars in the MW disk. Using high-resolution numerical
simulations we estimate that the new space telescope Gaia should detect the
kinematic signatures of a few starless substructures provided the CDM paradigm
holds. Such a measurement will provide unprecedented constraints on the
primordial matter power spectrum at low-mass scales and offer a new handle onto
the particle physics properties of dark matter.
