A low-surface-brightness galaxy (LSB galaxy) is a galaxy that is dimmer than what is usually seen and catalogued. Once the brightness of observed galaxies was characterized, it was theorized that there might be dimmer galaxies that were less likely to be noticed, i.e., a selection bias, and this proved to be the case. They include galaxies as dim as the surrounding sky, or more.
With this discovery, the term high-surface-brightness (HSB) galaxy is used in contrast with LSB galaxy to represent the type that was already known.
As a group, LSB galaxies show a high mass-to-luminosity ratio, suggesting a greater than usual percentage of its matter being dark. They are often dwarf galaxies. They also show few supernovae.