After a 20-year-long experiment, a team of international scientists detects the last of the missing intergalactic material predicted to be created by the Big Bang.

Specifically, the team was finally able to detect the missing parts of the "ordinary matter" that makes up everything in the universe, from the stars to the cores of black holes. This ordinary matter is different from the "dark matter" that comprised the bulk of the universe's mass. The dark matter remained to be undetected until now.

About two-thirds of this ordinary matter had been observed throughout the universe's first couple of billions of years. However, the rest of it had remained elusive until this present experiment.

To be precise, this ordinary matter was identified as the "baryons," the particles made from quarks that comprised the nuclei inside the atoms.

The team said that finding the missing baryons is important to finally put together the puzzles about how the universe began.

"This 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," said Michael Shull, one of the coauthors of the study, from the Department of Astrophysical and Planetary Sciences at the University of Colorado Boulder.

The Lost Baryons

In the 1990s, astrophysicists had estimated the amount of hydrogen and helium atoms created in the Big Bang. Accordingly, the yet-to-be-found dark matter comprised about 25 percent of the universe, while dark energy comprised about 70 percent. The ordinary matter or baryons was estimated to make up only about 5 percent of the universe.

Previous researchers concluded that 10 percent of the baryons end up in the galaxies, and about 60 percent end up in diffused clouds of gas, which are found in the spaces between galaxies. Still, there was about more than 30 percent left that have yet to be attributed in the universe. This unaccounted percentage was the focus of the current experiment published in the journal Nature on June 20.

Using the XMM-Newton from the European Space Agency, NASA's Chandra X-ray Observatory, and the Cosmic Origins Spectrograph on the Hubble Space Telescope, the team observed the Cosmic Microwave Background. It is the most ancient light in the history of the universe. Its age dates back to about 380,000 years after the Big Bang.

Finding The Missing Baryons

The team, led by Fabrizio Nicastro of the Harvard-Smithsonian Center for Astrophysics, first detected a "highly ionized gas" located between the quasar and the solar system.

Quasar is a massive galaxy with a supermassive black hole at its center. This black hole is actively eating matter and emitting X-rays and radio waves. Quasar's light takes more than 4 billion years to reach Earth.

By observing the quasar for 18 days, split between 2015 and 2017, the team found that the "highly ionized gas" they detected was at a high-enough density to be accounted for as the missing 30 percent of the ordinary matter or the baryons. To be exact, the baryons exist as filaments of oxygen gas at temperatures of around 1 million degrees Celsius, lying in the spaces between galaxies.

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