The recent observations of gravitational waves from cosmic events opens a dramatic new window on the universe (first in 2015). The first four events observed by gravitational waves were colliding black holes with masses between 7.5 and 35 solar masses. As black holes do not emit any radiation, there is no electromagnet radiation from the resultant black holes although the actual collision produces a gamma ray pulse which has been detected for one of the events. The amount of energy radiated away as gravitational energy from these black hole collisions is immense: between 1 and 3 solar masses of energy emitted in a short time of order or less than 100 seconds.
Even more interesting in terms of observations was the GW170817 event (published on 16 Oct 2017) which was caused by the collision of two neutron stars approximately 130 million years ago.
Observations by the W. M Keck Telescopes provide clear evidence for the existence of a supermassive black hole at the centre of our Milky Way galaxy. Because of dust, the galactic centre cannot be seen at visible, ultraviolet or soft x-rays wavelengths, but the W. M. Keck Telescopes have obtained incredible images of the stars and gas clouds at the centre of our Milky Way galaxy using infrared optics.