The hypothetical antithesis of a black hole, a white hole spews matter out. No antigravitational field has been discovered that actually produces this effect, however, so white holes remain purely speculative. One way wormholes could feasibly be generated is as a black hole that is attached to a white hole if they are found to exist.
Some speculation exists that quasars are actually white holes instead of supermassive black holes, but this is not widely accepted within the astronomical community.