D. Sluse

At the heart of every galaxy slumbers a supermassive black hole. When it devours matter, its surroundings flare up: this is an active nucleus. How can we learn about past outbursts if the black hole is silent? The answer lies in light echoes.

In the 1930s, Fritz Zwicky first noticed that galaxies in clusters were moving too fast — something invisible was holding them. Later, Vera Rubin measured the rotation of spiral galaxies, confirming that space is permeated by dark matter.