A blackhole is merely a large mass in space. It's mass is great enough that light (which has been proven to bend around matter) cannot escape it's force (having an escape velocity of more than 300,000kps/186,00mps).
As far as a white hole goes I'm not sure how you could have something which is the 'time reverse' of a black hole. The black hole doesn't exist in time in any special way. It simply is a big ball of matter that continues growing and eating more and more matter.
The theory that this matter 'emerges' somewhere is, as far as I know, rather unsupported. In theory if enough matter is poured into a blackhole it gets to a point where there is so much internal pressure that it explodes at phenomenal rates. In laymen's terms - a big bang.
What, may I ask, is a white hole? Based on your post I'd say we can't find them for the same reason we can't find the tooth fairy.
Okay - a little leveling here:
In exotic theory, matter entering a blackhole would reach or get infinitely close to the speed of light and thus be suspended in time infinitely (this is also known as a 'singularity' in some spatial science). Any matter that accelerates also slows down the time through which it passes, hence an object at the speed of light being completely stopped. I'm not sure I comprehend how you could have a white hole which is the 'time reverse'... you cannot move things in a 'negative' direction where time accelerates rather than decelerates.
AK - what on Earth do you mean?