The Strangest Black Holes in the Universe. Middleweight Black Holes. Scientists have long thought that black holes come in three sizes — essentially small, medium and large. Relatively small black holes holding the mass of a few suns are common, while supermassive black holes millions to billions of solar masses are thought to lurk at the heart of nearly every galaxy. One more massive than four million suns, for example, is thought to hide in the center of the Milky Way.
However, middle-weight black holes had eluded astronomers for years. Scientists recently discovered an intermediate-mass black hole, called HLX-1 (Hyper-Luminous X-ray source 1), approximately 290 million light-years from Earth, which appears to be about 20,000 solar masses in size.
Medium-size black holes are thought to be the building blocks of supermassive black holes, so understanding more about them can shed light on how these monsters and the galaxies that surround them evolved.https://www.space.com/15941-strangest-black-holes-universe-countdown.html