Artist's depiction of a Worm hole
Added to this, if black holes exist in the way general relativity leads us to suspect, it is doubtful they can be used as ‘gates’ to enter wormholes and get to another part of the cosmos. The reason is that the enormous gravitational pull exerted by a black hole would rend asunder any object in its vicinity. A space ship, therefore, would be ripped to shreds long before it could enter a black hole.
Finally, the whole theory of white holes has fallen more and more into disfavor. We now since found - by way of more recent observations- that quasars could simply be the ejected cores of energetic galaxies. So, the same source that powers exploding galaxies, could also power them without invoking anything exotic like white holes.