Mystery of the Universe's Missing Matter Finally Solved by Scientists

andromeda milky way
The Milky Way and Andromeda galaxies. NASA

The universe is missing half its matter. Scientists have been faced with this cosmological problem for many years—there is a huge imbalance between how much we see and how much our models say should be there.

Scientists believe most of the universe—about 70 percent—is made up of dark energy, the mystery force thought to be driving its expansion. About a quarter is dark matter, and the remaining five percent is normal matter—the physical substance that makes up everything we can see in the universe, including planets, stars and galaxies.

However, our observations of normal matter (protons, neutrons and electrons) only account for about 2.5 percent of the universe—the rest of it is nowhere to be found. This mismatch is known as the "missing baryon problem." Two teams of researchers have now claimed to have resolved this issue.

According to New Scientist, Hideki Tanimura at the Institute of Space Astrophysics in Orsay, France, and Anna de Graaff, from the University of Edinburgh, U.K., have in two separate papers found the missing baryons in the hot filaments of gas that link galaxies together.

The teams looked at galaxies mapped in the Sloan Digital Sky Survey predicted to be connected by baryons. They then took data from the Planck satellite, which maps the cosmic microwave background (CMB)—the afterglow from the Big Bang. As this light moves through hot gas, some of it scatters, leaving a patch in the CMB. This phenomenon allowed the researchers to see strands of matter that are normally far too dim to observe.

Both teams, looking at over a million pairs of galaxies, found evidence of gas filaments in the space between galaxies. Furthermore, they found the matter was far denser than average—in Tanimura's paper it was up to three times denser, while in Graaf's paper it was as much as six.

"The missing baryon problem is solved," Tanimura told the magazine. "We expect some differences [between the density] because we are looking at filaments at different distances. If this factor is included, our findings are very consistent with the other group."

Ralph Kraft, from the Harvard-Smithsonian Center for Astrophysics in Massachusetts, said the findings help align the discrepancy between observations and simulations of the universe. He said the studies go "a long way" in showing that many of our fundamental ideas about space appear to be right. "Everybody sort of knows that [the missing matter] has to be there, but this is the first time that somebody—two different groups, no less—has come up with a definitive detection," he said.

Tanimura's paper has been submitted for publication in the Monthly Notices for the Royal Astronomical Society, while de Graaff's has been submitted to the Nature journal.