Gravitational lensing by dark galactic haloes

Recent work on galaxy formation scenarios has raised the interesting possibility that there could be many galactic halo-like objects in which luminous galaxies have failed to form. It is considered here whether dark galactic haloes which lack compact luminous components will reveal themselves by multiply imaging distant quasars. A single such dark halo is unlikely to do so, but two dark haloes coming along the line-of-sight can form multiple images of distance sources. If the density in such dark haloes is a significant fraction of the closure density of the Universe, such double halo lensing is found to be on the threshold of detectability. (author)