Cylon homeworld
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The planet which the Cylons established as their main base after rebelling against the Twelve Colonies of Kobol, and then fleeing into space to strike out on their own after the first Cylon War.
The Colonials apparently know the location of the planet the Cylons have established as their "homeworld". ("The Hand of God")
The fleet led by Galactica fled so far away from the Twelve Colonies that they exceeded the finite range of humanoid Cylon consciousness-download tranmissions back to the Cylon homeworld. As a result, the Cylons built the Resurrection Ship so Cylons engaging the human fleet would be able to be reborn when they died. ("Resurrection Ship, Part I")