Observations of quasar absorption lines show that intergalactic clouds were more common in the past.

Question

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The observation of quasar absorption lines indicates that intergalactic clouds were more abundant in the past. This is because the light from quasars travels through these clouds and is absorbed by the atoms and molecules present in them, leaving characteristic absorption lines in the spectrum of the quasar. By studying these absorption lines, astronomers have inferred that intergalactic clouds were more prevalent at earlier times in the universe's history.

Observations of Quasar Absorption Lines Show That Intergalactic Clouds Were