After years of searching, scientists can finally account for all the normal matter in the universe

Astronomers using a powerful quasar to study an enormous invisible tendril full of superheated gas say they may have finally discovered the universe's 'missing' detectable matter.

The findings, published in the journal Nature, solve a decades-old mystery and could help scientists further probe the structure and evolution of the cosmos.

All of the atoms in the stars, galaxies and planets in existence make up roughly 5 percent of the mass-energy density of the cosmos. The overwhelming majority, about 70 percent, is made up of dark energy—a mysterious, repulsive force that is causing the universe to expand faster and faster. The remaining quarter or so is made up of dark matter—invisible, untouchable stuff whose presence can only be felt by its gravitational influence on galactic scales. Dark matter connects clusters of galaxies with massive tendrils, forming a cosmic web that serves as an unseen skeleton for the universe.

Scientists have estimated those shares largely using two different methods, said study co-author J. Michael Shull, an astrophysicist at the University of Colorado, Boulder. Many years ago, researchers calculated roughly how much matter would have formed in the wake of the "big bang" that gave birth to the universe. Astronomers have also studied the cosmic microwave background – the oldest light in the universe, which permeates the entire sky – and found roughly the same proportions of normal matter, dark matter and dark energy.

That small slice of normal matter that we can directly detect, which scientists call baryonic matter, is the most known quantity of the three: It emits light (like the sun) or reflects it (like the moon), making it visible to us or detectable by telescopes. And yet it also presents its own mystery, because for decades, scientists haven't been able to find all of it.

"Over 20 years ago people noted that if you added up all the starlight and all the mass in galaxies that goes with that starlight, you only get about 10 percent of that 5 percent of ordinary matter," Shull said. "So there was a 'missing matter' problem going back over 20 years: where is the gas, where are the baryons, that aren't collapsed into stars and galaxies?"

"That's why we worried about it," he added. "It really goes to the heart of key predictions in cosmology about the big bang."

Researchers have slowly chipped away at that gap by adding to the census all the hot, diffuse gas in the enormous halos of galaxies and even larger galaxy clusters. But they wondered if even more of the missing matter might actually be suspended in the enormous filaments of dark matter that make up the cosmic web.

Here's the problem with finding that missing matter: It would be mostly made out of hydrogen, the simplest element and by far the most abundant in the universe. When hydrogen atoms are ionized, they can become invisible to optical wavelengths, making them very difficult to detect.

Luckily, if a cloud of ionized hydrogen sits between Earth and a source of ultraviolet light, that hydrogen will absorb certain wavelengths, leaving a distinct chemical fingerprint that astronomers can detect once it reaches their telescopes. Shull and colleagues have been further adding to the census by finding this ionized gas.

The problem is that as the gas gets hotter and hotter—say, above a million degrees Kelvin—ionized hydrogen stops leaving a clear signal in ultraviolet. So for this paper the researchers also targeted much rarer oxygen ions, and searched for their fingerprint in X-rays, which are much higher-energy wavelengths of light.

The scientists used the European Space Agency's XMM-Newton X-ray space telescope to study the BL Lacertae quasar 1ES 1553+113, an active, supermassive black hole at the center of a galaxy. Quasars gobble up matter and shine brightly in many wavelengths of light, from radio waves to X-rays. These celestial lighthouses can basically back light the material that crosses the beam's path, just as a flashlight beam illuminates unseen motes of dust in the air.

Studying the chemical fingerprint of oxygen in the X-rays from the quasar light, the scientists found a large amount of extremely hot intergalactic gas – so much that they calculate that this gas could account for up to 40 percent of the baryonic matter in the cosmos, which could be enough to explain the missing matter.

The researchers think that these ions may have started out in the hearts of stars that went supernova, and were thrown out of their home galaxies by these explosive stellar deaths. They may have been superheated by shocks. Atoms need to interact with each other to radiate energy, and because the individual atoms in this sparse gas were so far apart, unable to touch each other, they remained extremely hot.

Taotao Fang of the Jiujiang Research Institute in China, who was not involved in the study, pointed to a few possible alternate explanations, including that the ionized gas signal may have come from within a galaxy rather than from intergalactic gas embedded in a dark matter filament.

Still, Fang wrote in a commentary, the findings "offer a tantalizing glimpse of where the elusive missing baryons have been hiding."

The next step, Shull said, is to repeat these observations using other quasars, to see if the share of baryonic matter that they found holds up in other parts of the sky.

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