None that we know of. It seems to be easier to detect black holes and even supermassive black holes than a single white hole. Naively one might have expected equal numbers of each. So far as I know, we don't really know if all of the supermassive holes are truly 'black' or if there are a couple of white holes in the mix!
Gravitationally, they would look about the same from a great distance. They would be the centers of a great outpouring of mater and energy if they were produced by matter flowing into a black hole in some other part of our universe, or perhaps from another universe entirely. Quasars and active galaxies have nuclei where supermassive black holes are producing tremendous amounts of energy, but these all look like accretion disks surrounding the black hole and funneling matter into them which then liberates the energy.
Copyright 1997 Dr. Sten Odenwald
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