The Dark Moon is a moon where Queen Nehelenia lived, sometime after she became evil and turned her subjects into Remless. It was not as nice a place to live as the Moon Kingdom. After Nehelenia tried to get the Golden Crystal to take over the Moon Kingdom, and was imprisoned in a mirror by Queen Serenity as a result, the Dark Moon Circus (Zirconia, the Amazon Trio, the Amazon Quartet, and the various Remless) traveled from the Dark Moon to Earth at the time of a solar eclipse.
When Queen Nehelenia plunged Earth into darkness, the Dark Moon eclipsed the sun.
After her defeat at the hands of Sailor Moon and Sailor Mini Moon, Nehelenia was reduced to an ugly old woman, so, in the shadow of the Dark Moon, she returned to her mirror to stay there forever, restoring her eternal youth, and returned to the Dark Moon, which was later banished from Earth by Sailor Moon's power, restoring light to Earth.