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.

• Pharos
• C-COSMOS

Martin Elvis, Maxim Markevitch