Astronomers have located a vast cache of previously unseen matter by probing the voids between galaxies with pulses of radio waves from distant explosions. More than three-quarters of the universe’s normal atomic matter is hiding in those vast intergalactic spaces as tenuous clouds of warm gas, the researchers report today in Nature Astronomy.
For more than 2 decades, astrophysicists have been troubled by the “missing baryon problem”: the mismatch between the mass of all the stars, galaxies, and gas clouds they can see and the total amount of baryons—a class of particles including neutrons and protons—that they believe the big bang produced. The radio pulses, called fast radio bursts (FRBs), have now led to a full accounting of baryonic matter: Seventy-six percent is in warm intergalactic clouds, 15% is cold gas in and around galaxies, and just 9% makes up all the stars and planets. “I would say that the missing baryons problem is essentially solved,” says Nicolás Tejos, an astronomer at the Pontifical Catholic University of Valparaíso who was not part of the study. “Thanks to FRBs, we have now been able to close this baryon budget.”
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