‘Stupendously large’ black holes can’t be most of dark matter, study finds

Could dark matter be hiding inside a handful of colossal black holes? A new study says no, at least not for the biggest kind. Black holes heavier than entire galaxies cannot make up a significant share of dark matter, according to research reported by Space.com.

The idea being tested

Dark matter is the unseen material thought to hold galaxies together. It makes up about five-sixths of all matter, yet nobody knows what it is (our dark matter explainer covers the evidence and the main suspects). Black holes are natural candidates: they are massive and very hard to see.

The team looked at "stupendously large black holes", objects above 100 billion solar masses. The heaviest would weigh between a million billion and ten million billion suns, more than whole galaxies. None has ever been detected; the largest known black hole may weigh about 36 billion suns. That already ruled them out as all of dark matter, so the question was whether they could still be a large part of it.

Reading the universe's baby picture

Dark matter is usually imagined as "a fine dust spread everywhere," co-author Christopher Gerlach of the Mainz Institute for Theoretical Physics told Space.com. If it were instead packed into a few giant objects, "the universe becomes grainy on very large scales. And we can check that."

The check uses the cosmic microwave background, light released about 380,000 years after the Big Bang, when the cooling universe first let light travel freely. Its temperature varies by only about one part in 100,000 across the sky, and that pattern records how matter was spread out at the time.

The CMB does not show the graininess that a population of stupendously large black holes would cause. "So these black holes can't be a significant part of the dark matter," said co-author Nicholas Leister.

What it does and doesn't mean

• The result does not say such black holes can't exist, only that they can't be a major ingredient of dark matter.
• A very different candidate is still open: tiny black holes about the mass of an asteroid. The team is now working on whether those can be ruled out too.
• The paper, by researchers in Mainz and at New York University Abu Dhabi, is due to be published in Physical Review Letters on 26 October.

Searches for dark matter come at it from many angles. Earlier this month a radio survey turned up a starless hydrogen cloud that may be a dark galaxy, and astronomers are watching a pair of supermassive black holes on course to merge.