Only 31 percent of the universe is made up of matter, scientists have discovered, with the rest being made up of dark energy.
Previous models and observations of the universe have estimated that the universe is comprised mainly of dark energy and dark matter, with around 68 percent dark energy, 27 percent dark matter, and roughly 5 percent "normal" matter, according to NASA.
This has now been confirmed using a matter-measuring technique called mass-richness relation (MRR), according to a new paper in The Astrophysical Journal.
Dark energy was first theorized to exist as an explanation of why the universe is expanding at an accelerated rate, acting on a universe-wide scale. Dark matter, on the other hand, is a form of matter that does not interact with the electromagnetic field or with "normal" matter, making it unobservable by our current means, except by spotting its influence via gravity.
"Dark matter is a long-standing problem," physicist and astronomer Tommi Tenkanen previously told Newsweek. "We know it exists but not what it actually is nor what its origin is."
Exactly what dark matter and dark energy are is still unclear to scientists, but the observed properties of the universe don't make sense without them being taken into account.
"Cosmologists believe that only about 20 percent of the total matter [both "normal" and dark] is made of regular or 'baryonic' matter, which includes stars, galaxies, atoms, and life," Mohamed Abdullah, lead author of the paper and researcher at the National Research Institute of Astronomy and Geophysics in Egypt, and Chiba University, Japan, said in a statement. "About 80 percent is made of dark matter, whose mysterious nature is not yet known but may consist of some as-yet-undiscovered subatomic particles."
In the paper, the authors describe how they measured the amount of matter—both dark matter and regular matter as we know it—by comparing galaxy clusters to predictions of the clusters based on dark matter models.
"The team used a well-proven technique to determine the total amount of matter in the universe, which is to compare the observed number and mass of galaxy clusters per unit volume with predictions from numerical simulations," co-author Gillian Wilson, a professor of physics and vice chancellor for research, innovation, and economic development at the University of California Merced, said in the statement. "The number of clusters observed at the present time, the so-called 'cluster abundance,' is very sensitive to cosmological conditions and, in particular, the total amount of matter."
Since dark matter cannot be directly observed, this posed an issue in terms of measuring the matter.
"A higher percentage of the total matter in the universe would result in more clusters being formed," Anatoly Klypin, co-author and astrophysicist from the University of Virginia. "But it is difficult to measure the mass of any galaxy cluster accurately as most of the matter is dark, and we cannot see it directly with telescopes."
The scientists used the MRR method, which indirectly determines the mass of a cluster by assuming a more massive cluster contains more galaxies than less massive clusters, helped by spectroscopy, to determine the distance to each cluster and which galaxies were truly within each cluster. They could then compare these assumed masses to predictions based on models, finding that the universe is likely comprised of 31 percent total matter, which corroborated independent cosmic microwave background (CMB) observations from the Planck satellite.
"We have succeeded in making the first measurement of matter density using the MRR, which is in excellent agreement with that obtained by the Planck team using the CMB method," Tomoaki Ishiyama, co-author and researcher at Chiba University, said in the statement. "This work further demonstrates that cluster abundance is a competitive technique for constraining cosmological parameters and complementary to non-cluster techniques such as CMB anisotropies, baryon acoustic oscillations, Type Ia supernovae, or gravitational lensing."
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