Over the past century astronomers have discovered huge numbers of galaxies within our Universe by detecting the light from the stars they are made of. Now some astronomers believe that there are many other undetected galaxies that do not contain stars. IAU Symposium 244 saw heated debates surrounding the existence of these Dark Galaxies, discussing what a Dark Galaxy is and whether detections satisfy requirements for what one might be. Other contributions describe candidate Dark Galaxies, the baryon content of the Universe, and the discovery of the warm inter-galactic medium and low luminosity galaxies. Highlights include state-of-the-art observations of atomic hydrogen in the Universe that may be used to find Dark Galaxies, how gravitational lensing may be used to map the distribution of matter and inferences of dark structures, and the role of low luminosity galaxies in the formation of larger galaxies.
