The Intergalactic Medium We have already seen that intracluster matter exists in large quantities, apparently as a result of early galaxy evolution. It further seems unlikely that galaxy formation would have been 100% efficient by now; there might well be a great deal of matter in diffuse form spread throughout the universe. We are dealing here with "normal" baryonic matter, not exotic forms that manifest themselves mostly by gravitation. The relevant observational tests are: searching for neutral hydrogen and helium absorption in the spectra of high-redshift quasars. the diffuse X-ray background radiation. (This turns out to be dominated by faint discrete sources rather than a diffuse gas). searches for systematic dust reddening of very distant objects. (Again, colors of high-redshift objects allow very little scope for widespread intergalactic dust). evidence for external confinement of diffuse plasmas. The premier test for neutral gas as a component of the IGM is the
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