A particularly faint galaxy with a surface brightness at or below the brightness of the night sky. Current theories point to the idea that low surface brightness galaxies (LSBG's) may be some of the least evolved objects known to astronomy, consisting of large masses of diffuse gas with few bright stars.

LSBG's may constitute the majority of mass in the universe, but they were only discovered some 20 years ago when people started developing new methods of scanning through sky survey pictures. These new methods increased the contrast of the images, allowing dim galaxies to stand out. Malin 1, the first galaxy found this way, is currently considered to be the largest known object in the universe.

Current research into LSBG's is being conducted through study of the neutral hydrogen emissions from the galaxies as well as through optical CCD surveys. In part, LSBG's are of particular interest because the existence of such unevolved objects has implications on the way we view the evolution of the universe. LSBG's have been seen to deviate from normal galaxy behavior in a number of ways, falling off of the standard Tully-Fisher relation and demonstrating abnormally high mass-to-luminosity ratios.