The bulk of cosmic matter resides in a dilute reservoir that fills the space between galaxies, the intergalactic medium (IGM). The history of this reservoir is intimately tied to the cosmic histories of structure formation, star formation, and supermassive black hole accretion. Our models for the IGM at intermediate redshifts (2≲z≲5) are a tremendous success, quantitatively explaining the statistics of Lyα absorption of intergalactic hydrogen. However, at both lower and higher redshifts (and around galaxies) much is still unknown about the IGM. We review the theoretical models and measurements that form the basis for the modern understanding of the IGM, and we discuss unsolved puzzles (ranging from the largely unconstrained process of reionization at high z to the missing baryon problem at low z), highlighting the efforts that have the potential to solve them.

中文翻译：

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星际介质的演化

大部分宇宙物质存在于填充星系之间空间的稀释水库中，即星系间介质 (IGM)。这个水库的历史与结构形成、恒星形成和超大质量黑洞吸积的宇宙历史密切相关。我们在中间红移 (2≲z≲5) 的 IGM 模型取得了巨大的成功，定量地解释了星系间氢的 Lyα 吸收统计数据。然而，无论是在较低红移还是较高红移（以及星系周围），关于 IGM 的许多内容仍然未知。我们回顾了构成 IGM 现代理解基础的理论模型和测量，并讨论了未解决的难题（从在高 z 时基本上不受约束的再电离过程到低 z 时丢失重子问题），