  There is a large and growing interest in observations of small-scale
structure in dark matter. We propose a new way to probe dark matter structures
in the $\sim 10 - 10^8 \, M_\odot$ range. This allows us to constrain the
primordial power spectrum over shorter distances scales than possible with
direct observations from the CMB. For $k$ in the range $\sim 10 - 1000 \, {\rm
Mpc}^{-1}$ our constraints on the power spectrum are orders of magnitude
stronger than previous bounds.
  We also set some of the strongest constraints on dark matter isocurvature
perturbations. Our method relies on the heating effect such dark matter
substructures would have on the distribution of stars in an ultra-faint dwarf
galaxy. Many models of inflation produce enhanced power at these short distance
scales and can thus be constrained by our observation. Further, many dark
matter models such as axion dark matter, self-interacting dark matter and
dissipative dark matter, produce dense structures which could be constrained
this way.
