The ideal background «lights» for such a study are quasars, which are very distant bright cores of active galaxies powered by black holes. (sciencedaily.com)
Because the gas in the halo is dark, the team measured it by using the light from quasars, the very distant bright cores of active galaxies powered by black holes. (sciencedaily.com)
The two black holes live roughly 3.7 billion light - years away in a quasar, the ferociously bright core of a galaxy lit up by... (sciencenews.org)