No problem with the double effort. I will be patient, don't worry ^^
Dark matter was initially postulated to explain the motion of the stars in the galaxies, as well as the global motion of the galaxies themselves. Assuming gravity, observations are only reproduced if we add something invisible (that generates some gravitational force): dark matter.
Today, we also know that dark matter is also needed to explain galaxy formation, the cosmic microwave background, gravitational lensing.
In short, many cosmological observations are pointing towards its existence and everything seems to agree with each other. The only missing link is the direct observation of dark matter on Earth.
For what I understand from your reply there is no sure measure of the weight of the black holes (because dark matter could be part of them) and therefore I was wrong assuming that the scientists have this problem "solved".
The mass of a black hole (and not the weight) can be inferred from the motion of the objects lying around it. This is thus another story, different from dark matter. We indeed observe how objects are attracted by the black hole or orbit around it. From there, one can extract the black hole mass.