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An ultra-faint dwarf galaxy is a faint Local-Group dwarf galaxy. To this point, all known are Milky-Way satellites. They have been discovered in recent surveys such as Sloan Digital Sky Survey. They are of interest 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.