Is it possible that white holes are hiding in plain sight? In the 1970s, Stephen Hawking demonstrated that when a white hole and a black hole are in thermal equilibrium with their surroundings, they absorb and emit the same amount of radiation - and thus it's impossible to tell them apart. So maybe some of the objects we think are black holes could be white holes in disguise.
But that's a very specific circumstance. Hsu decided to investigate how white holes might behave in a vacuum (i.e., in isolation), when they would not be in thermal equilibrium with their black hole counterparts.
Prevailing theory is that black holes evaporate slowly over time, gradually shedding matter in the form of "Hawking radiation," but the white hole would neither absorb or emit radiation when isolated in space.
What happens to a white hole that is thus, well, constipated? Hsu postulates that it must explode, thereby releasing huge amounts of energy: "quasithermal radiation." He concludes that stable white holes simply can't exist in empty space, and that's why we see no evidence for them. They most likely exploded into quasithermal radiation long before we had the tools with which to observe them.