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 ( catalog 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 ( \sim 200 km s ^ { -1 } ) 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 ( catalog 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 \sim 10 ^ { 8 } years ago with a perturber of \sim 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 Based on observations made with the NASA/ESA Hubble Space Telescope , obtained at the Space Telescope Science Institute , which is operated by the Association of Universities for Research in Astronomy , Inc. , under NASA contract NAS 5-26555 .
These observations are associated with program # 10562 . to search for stars associated with the H i and find none down to an I band surface brightness limit of 31.1 \pm 0.2 mag arcsec ^ { -2 } .