Altreen was a galaxy known to the Anschlasom. From the perspective of the Milky Way Galaxy, Altreen lay beyond Lazria. In turn, Vesra was beyond Altreen along the route to the distant Som. (VOY novel: The Eternal Tide)
Billions of years ago, Altreen was known to the Anschlasom. When a group of them passing through the Som stranded on the Mikhal Outpost 10,000 years ago, they carved their astronomical knowledge into stone and a cave. Altreen was depicted on an artifact, one in a series arranged in the Fibonacci sequence.
In August 2381, Federation Fleet Commander Afsarah Eden, Counselor Hugh Cambridge and the Doctor traveled to the Mikhal Outpost to investigate Eden's origin, which was linked to the Anschlasom. Looking on the artifacts, Eden remembered the names of Anschlasom constellations and galaxies. (VOY novel: The Eternal Tide)