Black hole (phenomenon)
A black hole is an astronomical feature that occurs when a massive star collapses into a extremely small singularity so dense than not even light can escape its gravatational field.
Transformers have encountered black holes on a number of occasions and they have frequently been show to function as portals between alternate universe:
A Quintesson ship with captured Autobots and Decepticons aboard passed through one to a universe where the color spectrum was reversed.
A specially outfitted Powermaster Optimus Prime journeyed into one to discover nucleon.
The bounty hunter Axer traveled through one from the Generation 1 continuity to the Robots in Disguise continuity.
In the Unicron Trilogy, Unicron's destruction created an especially large black hole which threatened to destroy the universe.
