The Unicron Singularity is a frightening black hole near Cybertron that threatens to swallow it, and eventually the rest of the universe, whole. The only way to destroy it and put the universe back in order is to gather up the four Cyber Planet Keys and the Omega Lock, hidden on various planets, and use them to awaken Primus.
The Unicron Singularity was created at the end of the Powerlinx Battles as a result of Primus' attempt to bury his eternal nemesis Unicron in the center of a newborn sun. This plan backfired, and Unicron's essence collapsed in on itself, creating a black hole. The black hole's pull was so strong even Unicron's presence in alternate realities was affected, and the fabric of space-time itself was increasingly unstable, altering even the memories of Optimus Prime and the Autobots under his command.
(Note: The Japanese version of Cybertron, Galaxy Force, made no connection between Unicron and the "Grand Black Hole." The Grand Black Hole was created instead by the destruction of an unnamed evil which disturbed the balance of the universe. Additionally, the planet Cybertron itself was swallowed up by the Grand Black Hole in Galaxy Force's first episode. In the American version, these scenes were omitted, and it was understood that the black hole hovered threateningly near the planet.)