Not all the original, primordial, gas originally spread almost perfectly
uniformly after the Big Bang has yet collapsed to form galaxies. Galaxy
formation is still ongoing, and we can watch it happening over cosmic time by
looking to high redshifts. The gas not yet condensed into galaxies is called
the "Intergalactic Medium". At early times the gas is cool, but more recently,
as the collapse begins, the gas heats up to near million degree temperatures,
so that almost the only signals of its presence lies in the X-ray band,
In today's universe most atoms should lie in this hot medium, not in galaxies,
stars or planets. CfA scientists are searching for this signal, but even the
X-ray signals are weak.
Martin Elvis, Maxim Markevitch
Spectrum of the blazar Markarian 421 in X-rays from Chandra.
Faint dark bands - too faint to be seen in this representation - at
specific wavelengths along this rainbow like spectrum reveal the
presence of tenuous hot gas in intergalactic space. Although far
closer to a true vacuum than the interplanetary space of our Solar
System, this gas fills such a large volume that in total it constitutes
almost half of all the atoms in our local Universe. (Nicastro et al. 2005,
Nature, 433, 495, February 3a).