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The term ultra-faint dwarf galaxy (UFD) was coined to describe some faint dwarf galaxies discovered in the Local Group, but examples have since also been detected somewhat beyond the Local Group. Currently, most known ultra-faint dwarf galaxies are Milky-Way satellites, uncovered in recent surveys such as Sloan Digital Sky Survey (SDSS). One reason they are of interest is because cosmology theory predicted more satellites of larger Milky-Way-like galaxies than had been observed in the past (the dwarf galaxy problem), and it has been suggested that the gap between observation and theory has closed somewhat.
The galaxies labeled UFDs are distinct from what are termed ultra diffuse galaxies and low-surface-brightness galaxies, terms applied to more distant galaxies.