  Recently a population of large, very low surface brightness, spheroidal
galaxies was identified in the Coma cluster. The apparent survival of these
Ultra Diffuse Galaxies (UDGs) in a rich cluster suggests that they have very
high masses. Here we present the stellar kinematics of Dragonfly 44, one of the
largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II
telescope. We find a velocity dispersion of 47 km/s, which implies a dynamical
mass of M_dyn=0.7x10^10 M_sun within its deprojected half-light radius of
r_1/2=4.6 kpc. The mass-to-light ratio is M/L=48 M_sun/L_sun, and the dark
matter fraction is 98 percent within the half-light radius. The high mass of
Dragonfly 44 is accompanied by a large globular cluster population. From deep
Gemini imaging taken in 0.4" seeing we infer that Dragonfly 44 has 94 globular
clusters, similar to the counts for other galaxies in this mass range. Our
results add to other recent evidence that many UDGs are "failed" galaxies, with
the sizes, dark matter content, and globular cluster systems of much more
luminous objects. We estimate the total dark halo mass of Dragonfly 44 by
comparing the amount of dark matter within r=4.6 kpc to enclosed mass profiles
of NFW halos. The enclosed mass suggests a total mass of ~10^12 M_sun, similar
to the mass of the Milky Way. The existence of nearly-dark objects with this
mass is unexpected, as galaxy formation is thought to be maximally-efficient in
this regime.
