We still do not know what dark matter is, but we can attack a line through an option. It is not, according to a theory proposed by the brilliant Stephen Hawking, a lot of small microscopic black holes.
In the most rigorous test of the theory to date, an international team led by researchers at the Kavli Institute of Physics and Mathematics of the Universe (IPMU) in Japan sought the telltale sign of tiny black holes, and the result was very damning.
Scientists were hunting for a special glow of stars in a nearby galaxy-the way light would appear to us if a black hole less than a tenth of an inch was passing in front of it.
A black hole of this size may seem ridiculous, but the concept actually stems from an elegant Hawking theory that was trying to solve our massive problem of dark matter.
The problem is this: Based on our observations of the gravitational forces at play in the Universe, we know that 85% of the mass out there is created by something we can not see and yet we do not directly detect.
In a 1971 article, Hawking expanded the theory of primordial black holes that had been proposed by scientists Yakov Borisovich Zel? Dovich and Igor Dmitriyevich Novikov in 1966.
Soon after the Big Bang, theory goes on, when the Universe arose, there might be regions of matter in the primordial soup that were denser than others – dense enough for gravitational collapse.
As the resulting black holes did not form in the stars, they could be much less massive – only 10-8 kg.
Now even a black hole has a lot of mass. A black hole with an event horizon of 0.1 millimeters in diameter would have a mass of more than 67 quintillion metric tons. So if there were a lot of those tiny black holes out there – and they had not evaporated because of Hawking's radiation – they could possibly explain the mass we can not see.
And if there were a lot of those black holes out there, running at the tremendous speeds calculated by Hawking, we could see them curving the light of the objects they move in front, an effect called a gravitational lens.
It is this effect that the team focused its efforts. Using Hyper Suprime Cam at the Subaru Telescope in Mauna Kea, Hawaii, researchers from Japan, India and the United States observed the entire Andromeda galaxy, capturing 190 consecutive images in a total of seven hours.
If a primordial black hole moves between us and a star, the star is expected to flash and light for a few minutes to hours, while the black hole's gravity increases its light.
The team predicted that the abundance of smaller black holes than the Moon needed to produce the dark matter effect would result in about 1,000 lens events.
But the observations produced only one potential event – which means that primordial black holes can account for no more than 0.1% of dark matter. (A study last year found that primordial black holes could not account for more than 40% of dark matter.)
Which, in fact, means that dark matter is not made of small black holes.
It is unlikely to be the last nail in the coffin of theory – after all, scientists like to be extremely thorough. But so far, primordial black holes are a no, and we have to look for dark matter elsewhere.
The research was published in Astronomy of Nature.