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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.
Dark Matter supposedly dominates the extragalactic, yet no totally dark structure of galactic proportions has ever been convincingly identified. Earlier (Minchin et al. 2005) we suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster
Dark matter interactions with massless or very light Standard Model particles, as photons or neutrinos, may lead to a suppression of the matter power spectrum at small scales and of the number of low mass haloes. Bounds on the dark matter scattering
Observations of the redshifted 21-cm signal (in absorption or emission) allow us to peek into the epoch of dark ages and the onset of reionization. These data can provide a novel way to learn about the nature of dark matter, in particular about the f
A recent observation points to an excess in the expected 21-cm brightness temperature from cosmic dawn. In this paper, we present an alternative explanation of this phenomenon, an interaction in the dark sector. Interacting dark energy models have be
Using the 21 cm line, observed all-sky and across the redshift range from 0 to 5, the large scale structure of the Universe can be mapped in three dimensions. This can be accomplished by studying specific intensity with resolution ~ 10 Mpc, rather th