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We present a new treatment of two popular models for the growth of structure,examining the X-ray emission from hot gas with allowance for spectral lineemission from various atomic species, primarily ``metals". The X-ray emissionfrom the bright cluster sources is not significantly changed from prior workand, as noted earlier, shows the CDM$+\Lambda$ model (LCDM) to be consistentbut the standard, COBE normalized model (SCDM) to be inconsistent with existingobservations --- after allowance for still the considerable numerical modellinguncertainties.  But we find one important new result. Radiation in the softer band 0.5-1.0keVis predominantly emitted by gas far from cluster centers (hence ``background").This background emission dominates over the cluster emission below 1keV andobservations of it should show clear spectral signatures indicating its origin.In particular the ``iron blend" should be seen prominantly in this spectral binfrom cosmic background hot gas at high galactic latitudes and should showshadowing against the SMC indicating its extragalactic origin. Certain OVIIlines also provide a signature of this gas which emits a spectrumcharacteristic of $10^{6.6\pm 0.6}$K gas. Recent ASCA observations of the X-raybackground tentatively indicate the presence of component with exactly thespectral features we predict here.Comment: Princeton University Observatory, in ApJ press, figs can be ftp'ed  from ftp://astro.princeton.edu/cen/XRAY

Background X-Ray Emission from Hot Gas in CDM and CDM+ Lambda Universes: Spectral Signatures