  We present results of our search for X-ray line emission associated with the
radiative decay of the sterile neutrino, a well-motivated dark matter
candidate, in Suzaku Observatory spectra of the Ursa Minor dwarf spheroidal
galaxy. These data represent the first deep observation of one of these extreme
mass-to-light systems and the first dedicated dark matter search using an X-ray
telescope. No such emission line is positively detected, and we place new
constraints on the combination of the sterile neutrino mass and the
active-sterile neutrino oscillation mixing angle. Line flux upper limits are
derived using a maximum-likelihood-based approach that, along with the lack of
intrinsic X-ray emission, enables us to minimize systematics and account for
those that remain. The limits we derive match or approach the best previous
results over the entire 1--20 keV mass range from a single Suzaku observation.
These are used to place constraints on the existence of sterile neutrinos with
given parameters in the general case and in the case where they are assumed to
constitute all of the dark matter. The range allowed implies that sterile
neutrinos remain a viable candidate to make up some -- or all -- of the dark
matter and also explain pulsar kicks and various other astrophysical phenomena.
