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Scientists have found what may be the universe鈥檚 lost sock at the back of the dryer鈥攁nswering a long-running mystery that astrophysicists have dubbed the 鈥渕issing baryon problem.鈥�

In a new study, an international team of researchers, including scientists from CU Boulder,听located the last reservoir of ordinary matter hiding in the universe. This matter, also called baryons,听makes up all physical objects in existence, from stars to the cores of black holes. But until now, astrophysicists had found only about two-thirds of the matter that theorists predict was created by the Big Bang. 听

To pin down the missing third, the researchers used the radiation emanating from a distant, ultra-bright black hole called a quasar. That lost matter exists as filaments of highly-ionized gas at temperatures of around 1 million degrees Celsius that lie in the space between galaxies, said CU Boulder鈥檚 Michael Shull, a co-author of the study.

Shull, of the Department of Astrophysical and Planetary Sciences (APS), said the results are important not just for completing the baryon problem but also for answering fundamental questions about how the universe began.听

鈥淭his is one of the key pillars of testing the Big Bang theory: figuring out the baryon census of hydrogen and helium and everything else in the periodic table,鈥� he said.听

The new study, which appears today in Nature, was led by Fabrizio Nicastro of the Italian Istituto Nazionale di Astrofisica (INAF)鈥擮sservatorio Astronomico di Roma and the Harvard-Smithsonian Center for Astrophysics.听

The results are the culmination of a 20-year search, Shull said. In the 1990s, astrophysicists came up with an estimate of how many hydrogen and helium atoms had been cooked up in the Big Bang. These baryons are distinct from the dark matter that makes up the bulk of the universe鈥檚 mass鈥攁nd that scientists have yet to find.听

Researchers think they know where most of that ordinary matter wound up: about 10 percent in galaxies and close to 60 percent in diffuse clouds of gas filling the vast spaces between galaxies. But that still left the census a little more than 30 percent short.

That鈥檚 where Shull and his colleagues came in. In 2012, he and Charles Danforth, a research associate at CU Boulder, called the warm-hot intergalactic medium (WHIM). It鈥檚 a wild terrain.

鈥淭his is where nature has become very perverse,鈥� Shull said. 鈥淭his intergalactic medium contains filaments of gas at temperatures from a few thousand degrees to a few million degrees.鈥�

To search for missing atoms in that perverse territory, the international team pointed a series of satellites at a quasar called 1ES 1553鈥攁 black hole at the center of a galaxy that is consuming and spitting out huge quantities of gas. 鈥淚t鈥檚 basically a really bright lighthouse out in space,鈥� Shull said. 听

Scientists can glean a lot of information by recording how the radiation from a quasar passes through space, like a sailor seeing a lighthouse through fog. First, the researchers used the Cosmic Origins Spectrograph on the Hubble Space Telescope to get an idea of where they might find the missing baryons in that fog. Next, they homed in on those baryons using the European Space Agency鈥檚 X-ray Multi-Mirror Mission (XMM-Newton) satellite.

The team found the signatures of a type of highly-ionized oxygen gas lying between the quasar and our solar system鈥攁nd at a high enough density to, when extrapolated to the entire universe, account for the last 30 percent of ordinary matter.

鈥淲e found the missing baryons,鈥� Shull said.

Shull said that the researchers will need to confirm their findings by pointing satellites at more bright quasars. He and Danforth, a co-author on the new study, will also explore how this gas got to these vast pockets of space. They suspect that it was blown out over billions of years from galaxies and quasars, but how is an open question.

鈥淗ow does it get from the stars and the galaxies all the way out here into intergalactic space?鈥� asked Danforth, who is also in APS. 鈥淭here鈥檚 some sort of ecology going on between the two regions, and the details of that are poorly understood.鈥�

The new study also includes authors from the SRON Netherlands Institute for Space Research; Instituto de Astronomia Universidad Nacional Autonoma de Mexico; University of Trieste; INAF鈥擮sservatorio Astronomico di Trieste; Istituto Nazionale di Fisica Nucleare; University of RomaTre; Princeton University; INAF鈥擮sservatorio di Astrofisica e Scienza dello Spazio di Bologna; Columbus State Community College; Ohio State University, Instituto Nacional de Astrof铆sica; Columbia University; INAF鈥擨stituto di Astrofisica e Planetologia Spaziali; Leiden Observatory and Instituto de Astrof铆sica de La Plata.