  Context: There is a possibility that the optically unidentified radio source
J1218+2953 may act as a gravitational lens, producing an optical arc ~4" away
from the radio position. Until now, the nature of the lensing object has been
uncertain since it is not detected in any waveband other than the radio. The
estimated high mass-to-light ratio could even allow the total mass of this
galaxy to be primarily in the form of dark matter. In this case, J1218+2953
could be the first known example of a "dark lens".
  Aims: We investigate the nature of J1218+2953 by means of high-resolution
radio imaging observations to determine whether there is a radio-loud active
galactic nucleus (AGN) in the position of the lensing object.
  Methods: We report on Very Long Baseline Interferometry (VLBI) observations
with the European VLBI Network (EVN) at 1.6 and 5 GHz.
  Results: Our images, having angular resolutions of ~1 to ~10 milli-arcseconds
(mas), reveal a rich and complex radio structure extending to almost 1". Based
on its radio spectrum and structure, J1218+2953 can be classified as a compact
steep-spectrum (CSS) source, and as a medium-size symmetric object (MSO). The
object harbours an AGN. It is also found as an X-ray source in the XMM-Newton
EPIC (European Photon Imaging Cameras) instrument serendipitous source
catalogue.
  Conclusions: Rather than being a dark lens, J1218+2953 is most likely a
massive, heavily obscured galaxy in which the nuclear activity is currently in
an early evolutionary stage.

Cosmology and Nongalactic Astrophysics