  Quasars are the visible signatures of super-massive black holes in the
centres of distant galaxies. It has been suggested that quasars are formed
during ``major merger events'' when two massive galaxies collide and merge,
leading to the prediction that quasars should be found in the centres of the
regions of largest overdensity in the early Universe. In dark matter
(DM)-dominated models of the early Universe, massive DM halos are predicted to
attract the surrounding gas, which falls towards its centre. The neutral gas is
not detectable in emission by itself, but gas falling into the ionizing cone of
such a quasar will glow in the Lyman-alpha line of hydrogen, effectively
imaging the DM halo. Here we present a Lyman-alpha image of a DM halo at
redshift 3, along with a two-dimensional spectrum of the gaseous halo. Our
observations are best understood in the context of the standard model for DM
halos; we infer a mass of (2-7) x 10^12 solar masses (Msun) for the halo.
