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For Decades, Some of the Atomic Matter in the Universe Had Not Been Located. Recent Papers Reveal Where It Has Been Hiding (wired.com)
In a series of three recent papers, astronomers have identified the final chunks of all the ordinary matter in the universe. From a report: And despite the fact that it took so long to identify it all, researchers spotted it right where they had expected it to be all along: in extensive tendrils of hot gas that span the otherwise empty chasms between galaxies, more properly known as the warm-hot intergalactic medium, or WHIM. Early indications that there might be extensive spans of effectively invisible gas between galaxies came from computer simulations done in 1998. "We wanted to see what was happening to all the gas in the universe," said Jeremiah Ostriker, a cosmologist at Princeton University who constructed one of those simulations along with his colleague Renyue Cen. The two ran simulations of gas movements in the universe acted on by gravity, light, supernova explosions and all the forces that move matter in space. "We concluded that the gas will accumulate in filaments that should be detectable," he said.
Except they weren't -- not yet. "It was clear from the early days of cosmological simulations that many of the baryons would be in a hot, diffuse form -- not in galaxies," said Ian McCarthy, an astrophysicist at Liverpool John Moores University. Astronomers expected these hot baryons to conform to a cosmic superstructure, one made of invisible dark matter, that spanned the immense voids between galaxies. The gravitational force of the dark matter would pull gas toward it and heat the gas up to millions of degrees. Unfortunately, hot, diffuse gas is extremely difficult to find. To spot the hidden filaments, two independent teams of researchers searched for precise distortions in the CMB, the afterglow of the Big Bang. As that light from the early universe streams across the cosmos, it can be affected by the regions that it's passing through. In particular, the electrons in hot, ionized gas (such as the WHIM) should interact with photons from the CMB in a way that imparts some additional energy to those photons. The CMB's spectrum should get distorted. Unfortunately the best maps of the CMB (provided by the Planck satellite) showed no such distortions. Either the gas wasn't there, or the effect was too subtle to show up. But the two teams of researchers were determined to make them visible. From increasingly detailed computer simulations of the universe, they knew that gas should stretch between massive galaxies like cobwebs across a windowsill. Planck wasn't able to see the gas between any single pair of galaxies. So the researchers figured out a way to multiply the faint signal by a million.
Except they weren't -- not yet. "It was clear from the early days of cosmological simulations that many of the baryons would be in a hot, diffuse form -- not in galaxies," said Ian McCarthy, an astrophysicist at Liverpool John Moores University. Astronomers expected these hot baryons to conform to a cosmic superstructure, one made of invisible dark matter, that spanned the immense voids between galaxies. The gravitational force of the dark matter would pull gas toward it and heat the gas up to millions of degrees. Unfortunately, hot, diffuse gas is extremely difficult to find. To spot the hidden filaments, two independent teams of researchers searched for precise distortions in the CMB, the afterglow of the Big Bang. As that light from the early universe streams across the cosmos, it can be affected by the regions that it's passing through. In particular, the electrons in hot, ionized gas (such as the WHIM) should interact with photons from the CMB in a way that imparts some additional energy to those photons. The CMB's spectrum should get distorted. Unfortunately the best maps of the CMB (provided by the Planck satellite) showed no such distortions. Either the gas wasn't there, or the effect was too subtle to show up. But the two teams of researchers were determined to make them visible. From increasingly detailed computer simulations of the universe, they knew that gas should stretch between massive galaxies like cobwebs across a windowsill. Planck wasn't able to see the gas between any single pair of galaxies. So the researchers figured out a way to multiply the faint signal by a million.
"First, the scientists looked through catalogs of known galaxies to find appropriate galaxy pairs — galaxies that were sufficiently massive, and that were at the right distance apart, to produce a relatively thick cobweb of gas between them. Then the astrophysicists went back to the Planck data, identified where each pair of galaxies was located, and then essentially cut out that region of the sky using digital scissors. With over a million clippings in hand (in the case of the study led by Anna de Graaff, a Ph.D. student at the University of Edinburgh), they rotated each one and zoomed it in or out so that all the pairs of galaxies appeared to be in the same position. They then stacked a million galaxy pairs on top of one another. (A group led by Hideki Tanimura at the Institute of Space Astrophysics in Orsay, France, combined 260,000 pairs of galaxies.) At last, the individual threads — ghostly filaments of diffuse hot gas — suddenly became visible."
So, when it is said "Missing" in this context they mean missing in the sense of predictions of the Standard Model with respect to the conditions of the early Universe when baryonic matter formed, correct?
Your first claim is correct.
It was missing only in that the standard model predicted it would be right where it is but we didn't before have sensitive enough equipment to detect it and prove it.
This just adds one more thing to the pile showing the standard model wasn't incorrect.
I can't say for either of the hypotheses "string theory" or "quantum loop gravity" you asked about, but I would assume no.
So far as predictions on the total mass from baryonic matter they already agreed with the other theories.
It's the non-baryonic and other stuff they tend to differ on, or I guess in the case of "other stuff" they tend to try and explain where the standard model doesn't.