A white hole is now a fairly accepted theory. What happens is when a super massive black hole (around the order of 10^15 solar masses) evaporates the evaportation accelerates. This evaporation is caused by virtual particle pairs and is explained in Hawking Radiation. The accelerating evaporation causes the black hole to start emitting more and more matter. As it reaches its death throes it starts to emit huge amouts of matter. This is the white hole. The only time a white hole of any magnitude would come to be is in a marginally bounded universe. This corresponds to the q0 term of Hubble's law being between 0 and 1/2. (This is assuming a cosmological constant of 0) Then afer all the mass in a galaxy collects into a super massive black hole some 10^31 years from now it is large enough to produce a white hole upon evaporation. A black hole this large would take something like 10^100 years to evaporate. Thus white holes can, and possibly do, exist but the time scale is far beyond the lifetime of the human race.