Question: Could black holes be made of dark matter?

Monique Henson answered on 16 Nov 2015:

In theory yes, but we don’t think they are normally. Black holes are usually created when stars die in supernovae. A supernova occurs when a star stops making heat and light and collapses in on itself. It leads to a huge explosion of light and matter. When really massive stars collapse, they can form a black hole. Since stars are primarily composed of normal (non-dark) matter, then these black holes should be made of normal matter.

However, black holes can in theory be made out of any kind of matter that is affected by gravity – which includes dark matter.

One problem for dark matter forming black holes (or even falling into them) is that dark matter often has too much momentum. We often think of things falling straight into black holes, but this isn’t what tends to happen. More commonly, things are drawn into orbit around black holes first. Then, objects lose momentum as they emit electromagnetic light (such as X-rays or gamma rays), which means their orbits start to decay and they start falling towards the black hole. Dark matter can’t emit light, so it’s orbit can’t decay in the same way. That makes it really hard for dark matter to fall into black holes.