We can identify neutral hydrogen clouds by their characteristic emission line at 21 cm.  Since neutral hydrogen is abundant in visible galaxies, it should be fairly abundant in dark galaxies as well.  So, while probing the Virgo galaxy cluster with the Lovell radio telescope, a large region of hydrogen was discovered.  The cloud was confirmed with the Arecibo radio telescope, but ccd imaging failed to reveal the source.  So, the oval indicates the angular extent of the object, and the calculated mass is 10^11 Mo.  For an object of this size and mass at that distance, we would expect to see a 12 magnitude object similar to the one on the right.