[Voltaire]

Thanks, Cooker. Presumably you mean that we can tell the mass of a black hole by reading the radiation spectrum at the accretion disk. Presumably this would look different based on the mass of the black hole...

But what about black holes that are not gobbling up anything? The only way we can measure their mass is by the gravitational effect they have on other objects. Consequently they could be part of the missing mass.

You assert that "Dark matter has nothing to do with black holes" and you suggest I read some books on cosmology. I have and I cannot explain why black holes are eliminated any better than you have done so far.

It should also be noted that some physicists believe it is possible that there is no missing dark matter, that the problem is that our model of gravity is at fault. Gravity may behave differently at the extremes misleading us into thinking there must be more mass than we can see.