My theory about Black holes is that they are made of Dark black matter. When normal matter enters the event horizon; they get transformed in to Dark Matter.
So my proposal here is that the Dark Matter is not a hallow around the galaxies but highly concentrated lumps( Black Holes) in the galaxies.
1. This explains the additional mass of galaxies
2. Black holes are black not because light can't escape it but there is no source that can produce light in a black holes