Scientists locate missing ordinary matter around galaxies

Astronomers have finally located the universe's missing 'ordinary' matter, solving a decades-old cosmic mystery. Using observations from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope, researchers identified diffuse clouds of hot gas surrounding galaxies, which account for the baryonic matter that had long eluded detection. This ordinary matter, made of protons and neutrons, was predicted by cosmological models but had not been directly observed in its expected quantities until now.

The discovery affects our fundamental understanding of the universe's composition. Baryonic matter—the stuff of stars, planets, and living beings—makes up only about 5% of the universe, with the rest being dark matter and dark energy. However, even within that small fraction, nearly half was unaccounted for in surveys of stars and galaxies. The newly found gas clouds, which are extremely diffuse and hot (around 1 million degrees Celsius), surround galaxies like the Milky Way and extend far beyond their visible disks.

This matters because it confirms a key prediction of the standard model of cosmology, which holds that baryonic matter should be distributed in a web of filaments between galaxies. The CHIME telescope, located in British Columbia, Canada, detected the missing matter by observing fast radio bursts (FRBs)—brief, powerful pulses of radio waves from distant galaxies. As these FRBs travel through the hot gas, their signals are delayed at certain frequencies, revealing the gas's presence and density.

Specific numbers from the study include that the missing matter was found in the circumgalactic medium (CGM) of galaxies, with densities consistent with theoretical expectations. The research, published in the journal Nature, used data from CHIME and the Canadian Hydrogen Intensity Mapping Experiment/Fast Radio Burst (CHIME/FRB) project. The team analyzed 13 FRBs to map the distribution of electrons along their paths, finding that the CGM contains enough baryons to account for the missing fraction.

Background: The search for missing baryons has been ongoing for over 20 years. Previous attempts using X-ray and ultraviolet observations had only found about half of the expected baryons in the local universe. The CHIME telescope, which began operations in 2017, was designed to map neutral hydrogen and detect FRBs, making it uniquely suited for this task.

Next steps: The researchers plan to use more FRBs to refine their measurements and map the missing matter in greater detail. Future telescopes like the Square Kilometre Array (SKA) could further confirm these findings and explore the role of the CGM in galaxy evolution. This discovery also opens new avenues for studying the large-scale structure of the universe and the processes that govern galaxy formation.