Black Hole merger

The mergers of black holes are the most energetic events observable in the Universe, but they are also invisible. Black hole mergers can only be detected by gravitational waves; the first of such merger was detected in 2015. The event in 2015, formally known as GW150914, produced three solar masses of energy in the form of gravitational waves in its final 20ms of merger, more energy than the combined electromagnetic energy of the observable universe put together.