Astronomers have long theorized the existence of dark galaxies, but there are no confirmed examples to date.
[8] Another way astronomers search for dark galaxies is to look for hydrogen absorption lines in the spectra of background quasars.
This technique has revealed many intergalactic clouds of hydrogen, but following up on candidate dark galaxies is difficult, since these sources tend to be too far away and are often optically drowned out by the bright light from the quasars.
In 2005, astronomers discovered gas cloud VIRGOHI21 and attempted to determine what it was and why it exerted such a massive gravitational pull on galaxy NGC 4254.
The Arecibo radio telescope was useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the 21-cm line.
These techniques have the capability of teaching us more about other special events that occur in the universe; for instance, the cosmic web.
The authors of the Hubble study suggested that one possible scenario was that the quasar is located in a dark galaxy.
Astronomer Josh Simon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass.
[17][18] J0613+52 is a possible dark galaxy,[19][20] discovered with the Green Bank Telescope when it was accidentally pointed to the wrong coordinates.
"[22][23] Smith's Cloud is a candidate to be a dark galaxy, due to its projected mass and survival of encounters with the Milky Way.