'Dark' galaxies contain no stars, making them impossible to observe using optical telescopes. Now, Sebastiano Cantalupo and his colleagues at the University of California, Santa Cruz, have managed to detect a faint fluorescent glow from a few such galaxies.

The researchers used the European Southern Observatory's Very Large Telescope to look at bright galaxies known as quasars, which can illuminate nearby dark galaxies. The team was able to see the glowing outline of a dozen candidate dark galaxies, thanks to radiation from a quasar exciting the hydrogen gas in these galaxies.

The team estimates the dark galaxies contain a gas mass around one billion times that of the Sun. These galaxies could serve as reservoirs of hydrogen fuel for star formation in larger galaxies, the authors suggest.

Mon. Not. R. Astron. Soc. http://dx.doi.org/10.1111/j.1365-2966.2012.21529.x (2012)