What is dark matter? There's still no definitive answer, but now scientists can say what it's not: a bunch of primordial black holes, as theorized by the great Stephen Hawking.
Dark matter is the mysterious substance that makes up 85 percent of the material universe. While its gravitational force is present throughout the cosmos, dark matter is unseen and undetectable.
For years, scientists have been trying to learn more about this elusive substance that's holding the galaxies together. So far, all efforts and instruments to find dark matter have come up short.
Hawking's Famous Theory
Hawking is one of these scientists, coming up with the famous theory that dark matter is made up of primordial black holes.
Shortly after the birth of the universe, the Big Bang may have created large amounts of miniscule black holes in regions that were dense enough for a gravitational collapse.
Still, even ultra-tiny black holes have great mass.
According to Science Alert, a black hole with an event horizon of 0.1 millimeters in diameter would have a mass of more than 67 quintillion metric tons. If plenty of these tiny primordial black holes exist in the universe, they could account for the great amounts of mass that's out there.
Additionally, if super-small black holes are zipping around in space, it would cause an effect known as gravitational lensing, which means the powerful gravitational field of black holes would bend the light of the objects that they move in front of.
It's this phenomenon that the new study uses to test Hawking's theory.
Astronomers Debunk Hawking's Dark Matter Theory
In new research detailed in the journal Nature Astronomy, an international team used the Hyper Suprime-Cam digital camera on the Subaru Telescope in Hawaii to hunt for primordial black holes in the neighboring Andromeda Galaxy.
If these primordial black holes were to move between Earth and a star in the nearby galaxy, the scientists would be able to spot the stars brighten and flicker in a quick "flash" as an effect of gravitational lensing. The smaller the black hole is, the faster the flash would be.
For these primordial black holes to produce dark matter, the scientists predicted that there would have to be enough black holes less massive than the moon to result in about 1,000 gravitational lensing events. The team was only able to spot one potential event out of 190 consecutive images over seven hours.
This single event is an achievement in itself, as Live Science notes that it is potentially the first ever detection of a primordial black hole.
It is a huge blow to Hawking's theory of dark matter being made up of these ancient black holes, though.
Lead author Masahiro Takada stresses that their research doesn't completely rule out the possibility of dark matter being made up of primordial black holes. However, they would have much, much smaller than expected.
"Our physicists are very excited because there is still a window," Takada told Live Science, adding that their data can't rule out the ultra-miniscule black holes whose flashes are much too short to detect. "So we need to think of another method to do it."