Even decades after it was first proposed by Vera Rubin and Kent Ford, dark matter remains a mystery. Something appears to affect galaxies but it doesn’t interact with light. We are much better at ruling out possible explanations than proving them right, and astronomers have just collected data that shows that dark matter is not made up of small primordial black holes.
The connection between the two is certainly appealing. The existence of primordial black holes is far from certain. Black holes and dark matter both interact exclusively with gravity and we only see their effects. The late Stephen Hawking and other physicists thought that primordial black holes were an intriguing explanation for dark matter. But unfortunately, this is not matched by observations.
Black holes are extremely dense objects, so dense that they can bend the light from objects they are in front of. If a huge number of black holes formed just after the Big Bang and are now making up dark matter, we might be able to spot subtle changes in the light of the galaxies they orbit. This phenomenon is called microlensing because it doesn’t warp the background light completely. It just gives the light of the star a little boost.
Researchers at the Kavli Institute for the Physics and Mathematics of the Universe used the Subaru Telescope to observe the Andromeda galaxy. To see a microlensing event, you need to have a black hole, a star, and the observers aligned. To maximize their chances, the team used the Hyper Suprime-Cam digital camera on the telescope, which can snap the whole of Andromeda in a single shot. They collected over 190 images of Andromeda.
If the dark matter was made by black holes just 0.1 millimeters in radius (so lighter than the Moon), the team estimated that they would see 1,000 candidate microlensing events. They observed only one.
The findings, reported in Nature Astronomy, imply that primordial black holes can only be responsible for no more than 0.1 percent of dark matter. Research from last year already showed that black holes can only account for less than 40 percent of dark matter. The mystery of the true nature of dark matter endures.