Sins of a Solar Empire

It is a running joke from the days of the beta testing for the game. Finding them does nothing aside from marking off the achievement from the list.

The easiest way to find them is to do a Huge Random map as the Advent. Pick a Mothership for first capital ship and focus on raising its colonizing bonus. Then get the Divination tech to reduce exploration costs. Then you can do planet exploration for free whenever the Mothership colonizes a planet. You should find them eventually.