Starts With A Bang

Why doesn’t dark matter form black holes? (Synopsis)

An illustration of a black hole. Despite how dark it is, all black holes are thought to have formed from normal matter, not dark matter. Image credit: NASA/JPL-Caltech.

“All enterprises that are entered into with indiscreet zeal may be pursued with great vigor at first, but are sure to collapse in the end.” -Tacitus

Dark matter makes up the overwhelming majority of mass in the Universe. With five times as much of it as there is normal matter, and the fact that it doesn’t emit or absorb light but does interact gravitationally, it might seem like the perfect candidate to form black holes. After all, it’s named “dark matter” to begin with, and black holes are perhaps the darkest form of matter in the whole Universe.

While stars might cluster in the disk and the normal matter might be restricted to a nearby region around the stars, dark matter extends in a halo more than 10 times the extent of the luminous portion. Image credit: ESO/L. Calçada.

But black holes are also extremely collapsed objects, while dark matter is incredibly diffuse. The physics of what enables an object to collapse isn’t governed by gravity, but rather by the other forces: the ones that enable energy transfer, momentum loss and inelastic collisions. As far as we can tell, those are properties that are unique to the particles of the Standard Model, and dark matter’s inability to interact in those lossy fashions means it cannot collapse.

The cosmic web is driven by dark matter, but the small structures along the filaments form by the collapse of normal, electromagnetically-interacting matter. Image credit: Ralf Kaehler, Oliver Hahn and Tom Abel (KIPAC).

No collapse means no black holes. That’s why dark matter might govern the great cosmic web, but can’t form the smaller bound structures we find within a galaxy.