Unknown

While the Federation has knowledge of thousands of species across thousands of worlds, there are still numerous encounters with aliens that there is no known information regarding their species. 
Prior to the Dominion War, the Chief of Security on Deep Space 9, Odo, was one such individual whose Changling race had not yet been discovered. Many individuals who do not know their species or their origins feel a deep longing towards finding out who they are and where they come from.