Viewing geometry determines whether the center of the active nucleus is hidden by an obscuring torus and whether jets of outflowing matter point nearly at us. If they do, they appear increased in brightness. Over the past 10 billion years quasars have dimmed by more than 100 times. The evidence that there may be dead quasars in the centers of normal galaxies includes the following.
Many galaxies have regions in their cores that resemble very weak quasars.
Also, massive but not very luminous objects have been found in the centers of several nearby galaxies. These methods all agree reasonably well and strongly require the presence of lots of dark matter in clusters. In the hierarchical clustering model, random fluctuations in the density of the Universe at very early times collapse gravitationally to form the structure we observe in the Universe. Small structures like galaxies formed first and then collected into the larger structures. The collapse of large structures like galaxy clusters has been so slow that it has taken the whole age of the Universe for it to happen.