  Many observations indicate that dark matter dominates the extra-galactic
Universe, yet no totally dark structure of galactic proportions has ever been
convincingly identified. Previously we have suggested that VIRGOHI 21, a 21-cm
source we found in the Virgo Cluster using Jodrell Bank, was a possible dark
galaxy because of its broad line-width (~200 km/s) unaccompanied by any visible
gravitational source to account for it. We have now imaged VIRGOHI 21 in the
neutral-hydrogen line and find what could be a dark, edge-on, spinning disk
with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has
unquestionably been involved in an interaction with NGC 4254, a luminous spiral
with an odd one-armed morphology, but lacking the massive interactor normally
linked with such a feature. Numerical models of NGC 4254 call for a close
interaction ~10^8 years ago with a perturber of ~10^11 solar masses. This we
take as additional evidence for the massive nature of VIRGOHI 21 as there does
not appear to be any other viable candidate. We have also used the Hubble Space
Telescope to search for stars associated with the HI and find none down to an I
band surface brightness limit of 31.1 +/- 0.2 mag/sq. arcsec.
