Where do medium-size black holes like to live?

I am a graduate student at the University of Sydney, studying black holes via their X-ray and radio emission during the day. I also work at Sydney Observatory at night. With telescopes we can observe matter falling into stellar-mass black holes (weighing a few times the mass of our Sun; formed when a massive star dies) and supermassive black holes (weighing more than a million times the mass of out Sun; lurking at the centres of many large galaxies, including our own Milky Way). But what happened to the black holes between these two mass extremes? Does nature not form them, or are they just hard to find? Come to my talk to find out the answer and much more!