(CNN) Mysterious fast radio bursts have been used to unlock another strange aspect of the universe: the case of the "missing matter."
Here's how: Fast radio bursts, called FRBs, are powerful millisecond bursts of radio waves that have been known to occur in distant galaxies and travel across the universe to reach Earth.
Astronomers have yet to determine what causes these fast radio bursts, which are unpredictable but can be spotted and traced back to their origin using sensitive telescopes.
But they were able to use six of them to detect all of the missing "normal" matter in the interstellar space between galaxies and stars. .
Normal matter, called baryonic matter in this study, is made of the protons and neutrons that comprise both humans and star stuff. But astronomers could only account for about half of it that should exist in the universe.
To be clear, this is not the detection of dark matter, which accounts for 85% of the universe but has yet to be observed or detected by astronomers. Dark matter is known by how it interacts with other matter, and the quest to find it continues.
The study published Wednesday in the journal Nature.
Unaccounted for, this missing matter was predicted to exist, but hard to find. In fact, astronomers have been searching for it over the last 30 years, the researchers said. Measurements of the Big Bang show how much matter was present in the early days of the universe, suggesting its existence.
"The result has squared the cosmic ledger," said Jean-Pierre Macquart, lead study author and associate professor at the Curtin University node of the International Centre for Radio Astronomy Research in Australia, in an email to CNN.
"Decades ago, astronomers had been able to infer how much matter existed in of the early universe, but up til now we were only able to account for roughly half of this in the present-day universe. Where had the matter gone?"
Macquart said it caused astronomers to question if their understanding of the early universe was defective, or if the matter disappeared or was just hiding in a state of very low density but high temperature that made it nearly impossible to find.
"It turned out that it was hiding in a density so low that it does not emit light, it doesn't absorb it, and it doesn't reflect it," he said.
It's there but not dense enough to be noticed. The sparse matter between stars is hard to find for this reason.
Astronomers first realized the potential of fast radio bursts to detect this missing matter the moment that their discovery was announced back in 2007, Macquart said.
"The radiation from fast radio bursts gets spread out by the missing matter in the same way that you see the colors of sunlight being separated in a prism," Macquart said.
Essentially, when the fast radio bursts pass through the missing matter, they slow down. The FRBs were used as a sort of "cosmic weigh station" for the missing matter, the researchers said. Surprisingly, even though they travel through quite a bit of matter to reach Earth, they act as a clean signal for the missing matter.
"Their millisecond durations made it very easy to measure the effect of dispersion — the process by which their longer wavelength emission is delayed with respect to their shorter wavelength emission is delayed — and hence to measure exactly how much matter they have encountered on their multi-billion year intergalactic journeys to Earth," Macquart said.
But the lead-up to this research was difficult. They needed to be able to identify the galaxy where each fast radio burst occurred, then point a telescope in that direction to determine the distance.
Astronomers were able to pin down the source of a repeating fast radio burst in 2017. But single radio bursts are harder to pinpoint because they don't reoccur.
The advent of the Australian Square Kilometre Array Pathfinder radio telescope, located in Western Australia, has enabled astronomers to essentially freeze the data from the short burst and use it to track back to the source.
When the burst arrives at the telescope, ASKAP records a live replay within a fraction of a second, according to Keith Bannister, principal research engineer at the Commonwealth Scientific and Industrial Research Organisation, or CSIRO, Australia's national science agency. He designed this pulse capture system.
"ASKAP both has a wide field of view, about 60 times the size of the full Moon, and can image in high resolution," said Ryan Shannon, study coauthor and associate professor at Swinburne University of Technology, in a statement. "This means that we can catch the bursts with relative ease and then pinpoint locations to their host galaxies with incredible precision.
Macquart said they were able to measure the distances to enough fast radio bursts to determine the density of the universe.
Celine Peroux, an astronomer at the European Southern Observatory who did not participate in this study, said the researchers have opened up a new field to study the properties of the missing matter. She also stressed that the role of the observatory's Very Large Telescope in Chile provided the key measurement of the distance of the bursts, which made the experiment possible.
"The measurement required the synergy of two very different kinds of telescopes in order to reach a firm conclusion," said Mariya Lyubenova, an astronomer at the European Southern Observatory not associated with the study, in an email to CNN. "The distribution of ordinary matter in the universe matters, because it determines the framework in which galaxies, stars, and ultimately planets form."
The detection also enabled further discoveries about fast radio bursts.
"We've discovered the equivalent of the Hubble-Lemaître Law for galaxies, only for fast radio bursts," Macquart said. "The Hubble-Lemaître Law, which says the more distant a galaxy from us, the faster it is moving away from us, underpins all measurements of galaxies at cosmological distances."
Next, the researchers want to use more fast radio bursts to determine how the matter has been distributed since they were able to measure it.
"Does it lurk in the general environs of galaxies, connecting them like thin wisps of candy floss? Or is it distributed uniformly, like a thin all-pervasive mist, throughout the vast expanses of intergalactic space?" Macquart said.
"We want to know this because this intergalactic matter plays a critical role in the cosmic ecosystem. It is like the 'atmosphere' in which galaxies are embedded, and it is critical to understanding how galaxies grow."