You wrote that
“you are forced to argue with your opponent using his terms and definitions”
OK, so I will concede partway to your definition of a black hole and will consider this invisible mass as a “blackkhole” that way it will have no colour and can be considered as a mass rather than a hole.
The question still remains; was Newton right or was Einstein right?
If Einstein was right then there needs to be no mass at the center of the galaxy, as long as the spacetime continuum is sufficiently curved, then the observations still hold true.
It is simply an assumption that mass “must” be present to create a curvature in the spacetime.