By Alvin Alexander. Last updated: December 15, 2017
“About 1.1 billion years ago, two black holes coalesced. They had danced around each other for billions of years, losing energy and swinging closer, until they could no longer resist each other's embrace. Spiraling in, they merged, and released a quaking shout of gravitational waves that echoed across the cosmos.”
Phil Plait on syfy.com has a nice story about a black-hole merger.