Ok that is a hard one…

It all starts with vacuum fluctuations cause a particle–antiparticle pair to appear.

See Wkipedia:

https://en.wikipedia.org/wiki/Quantum_fluctuation :

In quantum physics, a quantum fluctuation (or vacuum state fluctuation or vacuum fluctuation) is the temporary change in the amount of energy in a point in space,[1] as explained in Werner Heisenberg’s uncertainty principle. 

This allows the creation of particle-antiparticle pairs of virtual particles. The effects of these particles are measurable, for example, in the effective charge of the electron, different from its „naked“ charge.

If this happens near the event horizon of a black hole one partner of such a pair can fall into the black hole leaving the other one outside. This way the particle becomes real and could escape the black hole. It’s said that In order to preserve total energy, the particle that fell into the black hole must have had a negative energy.

Why…?

Why can’t it be the other way around and the particle which escapes has a negative energy? Taking away energy from the outside universe and the black hole would grow.

I never got that point why only partners with negative energy can fall into the black hole.

Willi Penker