Intergalactic Absorbtion Clouds
The transparent block schematically represents a column of space between a distant quasar and a telescope on earth. As the quasar's light travels across space, invisible, intervening clouds of hydrogen gas absorb certain frequencies of the light. This creates complex "thickets" of absorption features, schematically shown in the spectrum below the box. Ground-based observations have shown that the number of these clouds rapidly rises as one looks back in time.