Based on observations of distant gas clouds and galaxies, scientists know how much baryonic (normal) matter there was in the early universe. Surveys of closer (and therefore younger) regions of space turn up much less than the expected amount of matter. Taotao Fang of the University of California, Irvine and his fellow astronomers may have solved this mystery by locating a vast amount of matter in a hot diffuse cloud of gas.
The new gas cloud is called the Warm-Hot Intergalactic Medium (WHIM). Remember, astronomers are the guys who named the Very Large Telescope. In any case, the WHIM consists of the leftovers from galaxy formation and detritus from explosions. The reason it hadn’t been found before is that the WHIM is extremely diffuse and difficult to see. The WHIM only contains about six protons/cubic meter. Even the ‘empty’ space between stars in our galaxy contains about a million protons/cubic meter.
As Fang said:
Evidence for the WHIM has even been much harder to find than evidence for dark matter, which is invisible and can only be detected indirectly.
Fang and his colleagues were able to detect oxygen atoms in the WHIM as they absorbed X-rays created by a supermassive black hole. They compared data from both NASA’s Chandra X-ray Observatory and from ESA's XMM-Newton in order to be certain of their results.
Artist's rendition of WHIM (shown as a blue cloud surrounding a host of galaxies). An X-ray spectrum of the background source (shown as a beam of light in the larger picture) is given in the inset, where the yellow points show the Chandra data and the red line shows the best model for the spectrum after including all of the Chandra and XMM data. The dip in X-rays towards the right side of the spectrum corresponds to absorption by oxygen atoms in the WHIM.
(Credit: Spectrum: NASA/CXC/Univ. of California Irvine/T. Fang Illustration: CXC/M. Weiss)
Hat tip: Skeptics’ Guide to the Universe