Dark Matter supposedly dominates the extragalactic Universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Minchin et al. (2005) suggested that VIRGOHI 21, a 21-cm source found in the Virgo Cluster by Davies et al. (2004), was probably such a dark galaxy because of its broad line-width ( ˜ 200 km s-1) unaccompanied by any visible massive object to account for it. We have now imaged VIRGOHI 21 in the neutral hydrogen line, and indeed we find what appears to be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. We also find that there is an indubitable interaction with NGC 4254, a luminous spiral with an odd one-armed morphology but lacking the massive interactor normally linked with such a feature. Published numerical models of NGC 4254 call for a close interaction ˜ 108 years ago with a perturber of ˜ 1011 M&sun;. This we take as further, independent evidence for the massive nature of VIRGOHI 21. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation