Today, we know that almost every galaxy has a super-massive Black hole at its center. Before 1971, when the first black hole was discovered, black holes were considered to be theoretical objects. However, with sufficient evidences today, including the recent conformation of GRAVITATIONAL WAVES,
which were detected for the second time again few months after the first discovery in Sept. 2015, existence of black holes is confirmed.
White hole is a hypothetical object(region) in space-time that cannot be entered from the outside, although matter and light can escape from it. Some refer to them as reverse of a black hole. However, much like what black holes were few decades before, white holes still remain as a theory on papers. No white holes have been observed till date!
The Second law of Thermodynamics states that entropy of the universe can either increase or remain constant. White holes tend to reduce the entropy of the Universe- as matter can only escape from it and can never enter into it. If black holes were the epitome of disorder, sucking in everything in sight, a release of matter would be adding order, thus decreasing entropy.Therefore, in the case that a white hole did form, it would be highly unstable because it would be fighting the universe’s laws of physics, and would shortly collapse in on itself, perhaps turning into a black hole. Thus, their existence is a doubt.
Also, according to theory, a black hole would compress to the smallest possible size predicted by physics. After that, it is possible that, it may rebound as a white hole throwing all the matter and light outside that is trapped in it. However, due to time dilation effects, (time almost stops inside black hole) , it may take billions of years for even the lightest particle to escape the black hole. This would be the reason why many white holes may be stopped from forming if the arguments are true!
White holes can also be treated as the beginning of the universe as they are a source of matter and light and that no laws of thermodynamics were valid at the beginning of Universe!
White holes are just regions of mystery as of now, until we finally discover one and confirm its existence.