  The presence of unseen halos of ``dark matter'' has long been inferred from
the high rotation speeds of gas and stars in the outer parts of spiral
galaxies$^{1}$. The volume density of this dark matter decreases less quickly
from the galactic center than does that of the luminous mass (such as that in
stars), meaning that the dark matter dominates the mass far from the
center$^{1,2}$. While searching for faint starlight away from the plane of the
edge-on disk galaxy \gal$^{3}$, we have found that the galaxy is surrounded by
a faint luminous halo. The intensity of light from this halo falls less steeply
than any known luminous component of spiral galaxies, but is consistent with
the distribution of dark mass inferred from the galaxy's rotation curve.
