While black holes were collapsing stars sucking everything in to a central point left by the collapse and white holes were the opposite, “throwing all kinds of stuff out”, Grey Holes were something in between.
Grey Holes were extremely rare and should not have existed in the universe. When they did appear, they could break down the boundaries between dimensions. As well as being rare they were incredibly dangerous. They existed in a constant state of flux, creating different levels of space-time distortions. Entire galaxies and untold star systems could be contained in pocket universes. All of it is on the edge of oblivion. The guardians of the boundaries (the Boundary Nexus) were the Vorpon, Nexus Forms – the Grey Hole embodiments. (COMIC: The Grey Hole)