To describe the final stages of black hole evaporation will require a theory of quantum gravity, and no such theory exists at the moment. So your question cannot be answered: we simply don't know what happens when a black hole disappears.
I have seen a presentation (I'm afraid I don't have the link) where the final stages of evaporation were calculated using a string theory based model, and the evaporating black hole became a highly excited string. A quick Google finds various related articles such as this one, but I suspect the theory isn't well enough understood for these calculations to be more than speculation.
There are numerous misconceptions here, but allow me to address just a few:
Black holes do not have "appetite." In order for an object to be consumed by a black hole, the object's trajectory would need to literally intersect with the event horizon (i.e. be on a collision course with it), otherwise the object will continue to orbit the black hole. Because black holes are extremely compact, it actually makes it relatively difficult for orbiting objects to fall in. Instead, objects might come close to the black hole, and be accelerated to relativistic speeds, which accounts for the energetic phenomena that we observe in the vicinity of black holes.
All of this applies to Sgr A*. Despite how massive it is, it's also very compact. This makes it a relatively rare event to observe a star (or a gas cloud) actually falling into it.
We observe a large cloud of antimatter in the galactic center...
The "cloud of antimatter" to which you refer is not a cloud of antimatter, but a cloud of matter with a smattering of positrons that is slightly greater than elsewhere in the interstellar medium. It's also not quite centered on Sgr A*. For a much more complete answer on this subject see https://physics.stackexchange.com/a/111758/10334.
The universe is expanding with an accelerated speed. This requires energy to be added, and if energy pours in through white holes, energy is added.
...but we don't observe any energy "pouring in" from Sag A*. Furthermore, we know that the repulsive force of dark energy is uniformly distributed throughout space, and not localized to centers of galaxies.
We have never observed any singularity, so why should a black hole
singularity exist?
The singularity is, by definition, hidden inside the black hole, which is why we can never observe it.
Best Answer
Energy (in any form) falling into a black hole contributes to the mass of the hole, and mass is one of the many forms that energy can take, using the usual conversion factor: $E = mc^2$.