https://www.theregister.co.uk/2017/08/09/milk_way_100_million_black_holes/

A new study shows that there may be up to 100 million black holes scattered around the dark depths of the Milky Way – a number much higher than previously expected.

That figure was calculated by University of California at Irvine physicists, who took a closer look at the gravitational waves detected by the LIGO equipment in 2015. When massive stars collapse at the end of their lives, they turn into huge black holes, and when these hefty entities collide, they produce gravitational waves that are detected by LIGO on Earth. The team followed the source of those ripples back through the galaxy to estimate there could be millions of big black holes out there.