Thank you all for the valuable inputs. I would like to continue this discussion about the Dark matter and Black holes.
couple of reasons
1 The presence of Dark matter is identified by observing the effects. Which cannot pin point the location. It only gives a general idea of its presence
2 Number of Black hole in a galaxy or outside is again very difficult to accurately estimate. The type - Stellar Black holes for instance which can be approximately numbered is also very vague. for example the number in milky way could vary between 1 million to a billion ( according to one estimate ). This is just one type. So our understanding is not clear on the number of black holes in a Galaxy so how can we estimate the total mass and say that what we see cannot account for it all. The mass estimated by gravitational methods would be inclusive of the mass of all the black hole( which are known and many which are unknown) in that galaxy.