The familiar matter in the universe, known as baryonic matter, which is composed of protons, neutrons, and electrons, only makes up about 2.5 percent of the universe. The rest is nowhere to be found, a mismatch known as the "missing baryon problem." Two groups of scientists, however, appear to have resolved this issue.

Missing Baryons

In two separate papers, researchers reported finding the missing baryons in hot filaments of gas that link the galaxies together.

Researchers said that currently available instruments cannot directly observe this gas. The gas is very weak and not hot enough to be detected by X-ray telescopes so scientists were not able to see it before.

Sunyaev-zel'dovich Effect

To prove that these threads of gas indeed exist, researchers took advantage of the phenomenon known as Sunyaev-Zel'dovich effect, which causes a change in the apparent brightness of the Cosmic Microwave Background (CMB), the afterglow of the Big Bang.

As light travels, it scatters off the electrons present in the gas and leaves a dim patch in the CMB. The dim blotches, however, are far too weak to be directly seen so the researchers chose pairs of galaxies that they believe are connected by a strand of baryons.

The researchers then stacked the signals from ESA's Planck space telescope for the areas between the galaxies so individual faint strands become detectable as a group.

"Detecting the WHIM via the Sunyaev-Zel'dovich effect is extremely challenging due to its low density," commented Juan Macias-Perez, from the Laboratoire de Physique Subatomique et de Cosmologie in 2012, following Planck's discovery of the elusive web of gaseous filaments called warm-hot intergalactic medium (WHIM).

"The best chance to detect it is to look at the regions between pairs of nearby galaxy clusters that are interacting with one another: as they approach each other, gas in the inter-cluster region becomes denser and hotter, hence easier for us to spot," he adds.

Evidence Of Gas Filaments Linking Galaxies Together

After stacking a total of over a million pair of galaxies, the researchers found evidence of gas filaments linking the galaxies. The first group discovered that these filaments were nearly three times denser compared with the average of normal matter. The second group, on the other hand, discovered they were six times denser, which confirms that the gas is dense enough to form filaments.

"This result establishes the presence of ionized gas in large-scale filaments, and suggests that the missing baryons problem may be resolved via observations of the cosmic web," study researcher Anna de Graaff, from the University of Edinburgh, and colleagues wrote in their study.

The researchers said that some differences are expected because the two groups were looking at filaments at different distances.

"To our knowledge, this is the first detection of filamentary gas at over-densities typical of cosmological large-scale structure," wrote Hideki Tanimura, from the Institute of Space Astrophysics, and colleagues.

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