A hundred and thirty million years ago, two neutron stars in a galaxy far, far away crashed into each other and merged into a black hole. The impact was so large that here on Earth, 40 million parsecs away, we felt spacetime rumble.
How do we know? Well, two months ago we watched it live. The stars were so far away that it took 130 million years for the tremors to reach us - which they did on 17 August 2017.