Actually, I want to talk about the so-called Lyman-alpha forest. Suppose we look at a distant Quasar. It puts out a broad continuum in the UV, which gets red-shifted depending on how far it is away. On its way, it is extremely likely to encounter clouds of ionized intergalactic hydrogen, as well as more minor ingredients. Even at a cosmically "modest" red shift of 1, corresponding to a look back time of about 7.5 billion years, back in the strapping youth of the cosmos (5.5 Gyr.) there will be a forest of absorption lines in that emission. That forest consists almost entirely of differently red shifted versions of the Lyman-alpha line. The various incarnations are due to differently red shifted clouds of hydrogen along the path from quasar to Earth, with the least red shifted due to nearby clouds and the more red shifted to clouds farther away. The farther the source is away, the more intervening gas clouds until the "forest" becomes an impassible jungle of totally overlapping lines at higher red shifts. Beyond red shift 6 (to when the Universe was a mere toddler of less than 1 Gyr)nothing can be seen through the forest. There might have been enough neutral hydrogen around to suck up all the light, in those years when re-ionization was not yet complete.
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