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Published online by Cambridge University Press: 12 April 2016
A brief review of primordial helium and deuterium abundances suggests a baryonic mass density of ΩB ≈ 0.04 – 0.045 (for Ho = 70). This mass may be dominated by intergalactic gas in clusters and groups of galaxies. The observed low chemical abundances in evolved dwarf galaxies might suggest that outflow was the origin for such gas, and we make general suggestions for the interpretation of the data from the next generation of X-ray spectroscopic satellites. The effects of both outflow and inflow on the chemical evolution of galaxies is discussed, particularly in the context of low surface brightness galaxies, and we comment on their dust content.