Interactions with the Intergalactic Medium

In Chap. 2 it has been pointed that most of the baryonic matter of the universe is found in the intergalactic space (see, e.g., Table 2.1). Galaxies are known to interact with this intergalactic matter, and these interactions have important consequences for the IGM as well as for the galaxies. The interactions and their effects depend on the properties of the intergalactic matter. But in spite of its predominance of the baryonic mass, the intergalactic gas remains one of the less well known components of the cosmic matter. In the following paragraphs our limited knowledge on the high-redshift IGM properties will be briefly summarized.