In the original series, the metallic Cylons were a race of reptillians encased in metallic armour from the planet Cylon. You never got to the see the planet, you just heard about it. In the re-imagined series, the homeworld of the robot killing machines is Caprica as they were the creation of Graystone Industries. The planet never features in series and it is unknown as to whether the Cylons found a planet to make more copies of themselves or they just flew aboard spaceships where they produced more of their own.
Copyright: Vivendi Universal