Dark matter remains one of the biggest mysteries in physics. Scientists know the mysterious substance makes up the majority of all matter in the universe, vastly outnumbering normal matter like stars, planets, and people. But no one is sure exactly what it is.
A group of astronomers has at least narrowed the field of possible answers, though. They just ruled out a hypothesis about the nature of dark matter put forth by the late Stephen Hawking, who said that dark matter might be made of tiny black holes created at the beginning of the universe.
Astronomers and physicists are familiar with regular black holes, which are created when a giant star explodes and the remaining core is so massive it collapses under its own gravity. Hawking proposed a different and much smaller kind of black hole.
According to Hawking’s hypothesis, black holes could have formed in the early stages of the universe long before the first stars formed. In this era, all matter was free-floating clouds of gas, and some of that gas could have clumped together and collapsed into very small black holes. Because they didn’t form from stars, they might weigh less than a milligram.
If enough of these tiny black holes formed in the early years of the universe, they could possibly account for most or all of the undetected dark matter. Black holes are almost impossible to see directly, so that would explain why dark matter has remained so elusive for so long.
A group of astronomers at the Kavli Institute for the Physics and Mathematics of the Universe decided to test this theory by using the Subaru Telescope in Hawaii to look at the nearby Andromeda galaxy. If tiny black holes are out there somewhere, they reasoned, then some of them would be guaranteed to pass in front of some of the stars in Andromeda, distorting it slightly. That distortion could be spotted by the telescope.
Over a period of seven hours, the astronomers stared at Andromeda and hoped to catch the telltale flicker of light that signified a miniature black hole. If Hawking’s hypothesis was right, then you’d expect to see around a thousand such flickers. They saw one.
While this doesn’t prove beyond a shadow of a doubt that Hawking was wrong, it seems like a pretty convincing death sentence for his theory. That means we have to continue searching for the source of dark matter in strange exotic particles we’ve never seen. Perhaps this mystery will never be solved.