The Dream Cabbage is the grail of the ethically questionable Dr Mangle who first appears in "Lightning Man", and is also known as "Dr Mangle and his Dangerous Vegatables He knows of it's existence because he's seen it his formation orb.
Most his creations are fairly simple and do no more than leap out of shopping and give a nasty nip on the ankle.
The cabbage exists on an ethereal plane and is of a fractal nature. If you were to peel a leaf away and look closely at the veins and planes you would see more and more detail. The cabbage is an extraordinarily complex device as it is the energetic formation of the entire human races subconscious.
It is possible to have quantum interaction with the cabage should you for example wish to influence the dreams of anyone in particular. Guardian Robot is particularly adept at this as he is able to hold his own thoughts steady while filtering through the vastness. He also has isn't particularly interested in doing so.
The chaos would be irrevocable if Mangle ever got in there.