A massive outburst of energy,for a brief moment,it (collision of two black holes) outshine the entire observable universe.Scientists measured gravitational waves (which is caused by twisting space-time curvature)in LIGO and CERN where that waves can created by twisting space-time curvature by these two black holes.
For an example the collision of Milky way and Andromeda galaxy leads to a giant galaxy with a very giant black hole at the center.
The gravitational wave detectors include:
- LIGO (two facilities in the US)
- Virgo (in Italy)
- KAGRA (in Japan)…
- Image credit: The Guardian news