https://www.scientificamerican.com/article/readers-respond-to-the-february-2018-issue/

I was puzzled by one aspect of “The First Monster Black Holes,” Priyamvada Natarajan's article on the oldest supermassive black holes: If there were a substantial number of such black holes in the early universe, observable today at great distances, what happened to them? Would we not see them, and the quasars they produce, nearby in the modern universe?

Supermassive black holes have been found at the centers of spiral galaxies. Given that they are many times larger than previously thought, is “dark matter” still needed to hold those galaxies together?