  The recently discovered Ursa Major dwarf spheroidal (dSph) galaxy candidate
is about five to eight times less luminous than the faintest previously known
dSphs And IX, Draco, and UMi. In this Letter, we present velocity measurements
of seven color-magnitude selected UMajor candidate stars. Two of them are
apparent non-members based on metallicity and velocity, and the remaining five
stars yield a systemic heliocentric velocity of v=-52.45 +/-4.27 km/s and a
central line of sight velocity dispersion of 9.3 (+11.7 -1.2) km/s, with 95%
confidence that the dispersion is >6.5 km/s. Assuming that UMajor is in
dynamical equilibrium, it is clearly dark matter dominated, and cannot be a
purely stellar system like a globular cluster. It has an inferred central
mass-to-light ratio of M/L~500 and, based on our studies of other dSphs, may
possess a much larger total mass to light ratio. UMajor is unexpectedly massive
for its low luminosity -- indeed, UMajor appears to be the most dark-matter
dominated galaxy yet discovered. The presence of so much dark matter in UMajor
immediately suggests that it may be a member of the missing population of
low-mass galaxies predicted by the Cold Dark Matter (CDM) paradigm. Given the
weak correlation between dSph mass and luminosity, it is entirely likely that a
population of dark dwarfs surrounds our Galaxy.
