Universe’s ‘missing matter’ finally found in the space between galaxies

A fast radio burst (FRB) leaves its host galaxy as a bright burst of radio waves in this illustration. (Image credit: ICRAR)
A Hubble Space Telescope image of an FRB host galaxy, with the location of the FRB marked in red. This FRB was one of the network used to find the missing matter. (Credit: J. Xavier Prochaska/UC Santa Cruz, Jay Chittidi/Maria Mitchell Observatory, and Alexandra Mannings/UC Santa Cruz)
The FRBs were detected using CSIRO’s ASKAP radio telescope, which is located in outback Western Australia. (Image credit: CSIRO/Alex Cherney)
ASKAP measures the delay between the wavelengths of the FRB, allowing astronomers to calculate the density of the missing matter. (Illustration credit: ICRAR and CSIRO/Alex Cherney)
ASKAP continues to detect new FRBs, adding to the catalogue of these mysterious objects. (Illustration credit: ICRAR and CSIRO/Alex Cherney)

An international team of astronomers has solved the decades-old mystery of the ‘missing matter’ long predicted to exist in the universe but never before detected. The researchers have now found all of the missing ‘normal’ matter in the vast space between galaxies.

The discovery, published May 27 in Nature, was made by studying massive flashes of energy from deep space, called fast radio bursts.

“For decades, we have unsuccessfully searched for this missing matter with our largest telescopes. The discovery and localization of fast radio bursts was the key breakthrough needed to solve this mystery,” said corresponding author J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz.

Lead author Jean-Pierre Macquart at the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) in Australia said astronomers have been searching for the missing matter for almost 30 years.

“We know from measurements of the big bang how much matter there was in the beginning of the universe,” he said. “But when we looked out into the present universe, we couldn’t find half of what should be there. It was a bit of an embarrassment.”

Macquart said the missing matter was found lurking in the vast emptiness between stars and galaxies. “Intergalactic space is very sparse,” he said. “The missing matter was equivalent to only one or two atoms in a room the size of a normal office. So it's very hard to detect this matter using traditional techniques.”

The researchers were able to directly detect the missing matter using the phenomenon of fast radio bursts—brief flashes of energy that appear to come from random directions in the sky and last for just milliseconds. Scientists don’t know what causes them, but it must involve incredible energy, equivalent to the amount released by the sun in 80 years.

Macquart said the team detected the missing matter by using fast radio bursts as “cosmic weigh stations.”

“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,” he said. “We’ve now been able to measure the distances to enough fast radio bursts to determine the density of the universe. We only needed six to find this missing matter.”

The missing matter in this case is baryonic or ‘ordinary’ matter—like the protons and neutrons that make up all the elements in planets and stars. It’s different from dark matter, which remains elusive and accounts for about 85 percent of the total matter in the universe.

The research team also pinned down the relationship between how far away a fast radio burst is and how the burst disperses as it travels through the universe.

"We've discovered the equivalent of the Hubble-Lemaitre Law for galaxies, only for fast radio bursts," Macquart said. "The Hubble-Lemaitre Law, which was discovered in the 1920s, underpins all measurements of galaxies at cosmological distances.”

The fast radio bursts used in the study were detected using CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) telescope, which is located in outback Western Australia. ASKAP’s abilities were crucial to this study, said coauthor Ryan Shannon at Swinburne University.

“ASKAP has both a wide field of view, about 60 times the size of the full moon, and can image in high resolution,” he said. “This means that we can catch the bursts with relative ease and then pinpoint locations to their host galaxies with incredible precision.”

ASKAP is a precursor for the future Square Kilometre Array (SKA) telescope. The SKA could observe large numbers of fast radio bursts, giving astronomers a new way to study the previously invisible structure in the universe.