The baryon content of the universe is known from Big Bang nucleosynthesis and cosmic microwave background considerations , yet at low redshift , only about one-tenth of these baryons lie in galaxies or the hot gas seen in galaxy clusters and groups .
Models posit that these “ missing baryons ” are in gaseous form in overdense filaments that connect the much denser virialized groups and clusters .
About 30 % of the baryons are cool ( < 10 ^ { { 5 } } K ) and are detected in Ly \alpha absorption studies , but about half the mass is predicted to lie in the 10 ^ { { 5 } } -10 ^ { { 7 } } K regime , where detection is very challenging .
Material has been detected in the 2-5 \times 10 ^ { { 5 } } K range through OVI absorption studies , indicating that this gas accounts for about 7 % of the baryons .
Hotter gas ( 0.5-2 \times 10 ^ { { 6 } } K ) has been detected at zero redshift by OVII and OVIII K \alpha absorption at X-ray energies .
However , this appears to be correlated with the Galactic soft X-ray background , so it is probably Galactic Halo gas , rather than a cosmologically significant Local Group medium .
There are no compelling detections of the intergalactic hot gas ( 0.5-10 \times 10 ^ { { 6 } } K ) either in absorption or in emission .
Early claims of intergalactic X-ray absorption lines have not been confirmed , but this is consistent with theoretical models , which predited equivalent widths below current detection thresholds .
There have been many investigations for emission from this gas , within and beyond the virial radius of clusters , but the positive signals for this soft emission are largely artifacts of background subtraction and field-flattening .
We discuss the various techniques that can be used to detect the missing baryons and show that it should be detectable with moderate improvements in sensitivity .