An active galactic nucleus is a densely packed region of space at the heart of a galaxy that is considerably brighter than the rest of the galaxy.
The active nucleus is also a sign of stellar material such as gasses and dust being consumed by the black hole.
When the material falls towards the gravitational abyss, some of it ends up in a hot, spinning disc around the black hole.
The hot ring of material then releases radiation out into space, allowing astronomers to hone-in on the otherwise invisible black hole.
You can see from Hubble’s picture, the centre of the galaxy is much brighter than its darker swirls of stellar gas.