As I had suggested previously, temperature and thermodynamics are pretty much off topic In a thread that is nominally about friction. But you are the thread starter here so away we go...
If you simply Google "negative temperature", you can find
this article among others.
Recall the thermodynamic definition of temperature: ##\frac{\delta E}{\delta S}##. For almost any object we are accustomed to, this ratio is positive. The more energy you add to a system, the larger its state space becomes. It has more entropy.
Some exotic systems are such that their state space is eventually reduced as more and more energy is added. In this situation, the ratio ##\frac{\delta E}{\delta S}## is negative. This corresponds to a negative temperature.
If energy leaves such a system, that system's entropy increases. If that lost energy enters another system at a positive temperature, that system has its entropy increase as well. In accordance with the second law of thermodynamics, this means that heat will flow spontaneously from a body with a negative temperature to one with a positive temperature -- a negative temperature is "hotter" than all positive temperatures.
It is sometimes more convenient to use "inverse temperatures" like ##\frac{1}{T} = \frac{\delta S}{\delta E}##. The inverse temperature scale behaves consistently with respect to heat flow. Heat always flows from lower inverse temperature to higher inverse temperature.
[Corrections welcome from anyone who has actually taken a course in thermodynamics where this stuff is discussed. What I think I know, I've learned from hanging out here and Googling for background]As far as white holes go and a potential rescue of the universe from an eventual heat death, that sounds like wishful thinking layered on top of speculation. Let us not go there.