The Dark Zone is a black hole, located next to the planet Rugaro.
It is unknown how long the Dark Zone has existed, but by the time of "The Descent Into Darkness" it has grown big and strong enough to slowly pull Rugaro towards it, forcing the Beforians to try and secure the Dimensional Key that was on the planet. According to H.U.E., the Dark Zone first absorbs the complete spectrum of light, and then comes the descent into darkness.