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
The reason has to do with time dilation, and specifically, with the resulting red shift.
A black hole forms from a collapsing star, which is of course made of brightly glowing matter. The event horizon forms in the centre and moves outwards while the star-matter falls towards it. Because of gravitational time dilation, the infalling matter never crosses the event horizon from the outside perspective, and thus can technically still be "seen."
However, this time dilation also causes the light the matter emits to be redshifted. Essentially, every photon the matter emits is reduced in frequency due to the time dilation, and the time in between photons also reduces, asymptotically approaching infinity. This means that the black hole very rapidly converges to something that would appear completely black to an outside observer - the matter falling into it can only be seen by someone with the patience to collect many very low frequency photons over many billions of years.
That said, matter that falls into the black hole after it's been formed can certainly glow brightly enough to be detected - very much so in the case of an active galactic nucleus.