  About half a million years after the Big Bang, the ever-fading cosmic
blackbody radiation cooled below 3000 K and shifted first into the infrared and
then into the radio, and the smooth baryonic plasma that filled the Universe
became neutral. The Universe then entered a ``dark age'' which persisted until
the first cosmic structures collapsed into gravitationally-bound systems, and
evolved into stars, galaxies, and black holes that lit up the Universe again.
Some time between redshift of 7 and 15, stars within protogalaxies created the
first heavy elements; these systems, together perhaps with an early population
of quasars, generated the ultraviolet radiation that reheated and reionized the
cosmos. The history of the Universe during and soon after these crucial
formative stages is recorded in the all-pervading intergalactic medium (IGM),
which is believed to contain most of the ordinary baryonic material left over
from the Big Bang. Throughout the epoch of structure formation, the IGM becomes
clumpy and acquires peculiar motions under the influence of gravity, and acts
as a source for the gas that gets accreted, cools, and forms stars within
galaxies, and as a sink for the metal enriched material, energy, and radiation
which they eject. Observations of absorption lines in quasar spectra at
redshifts up to 5 provide invaluable insight into the chemical composition of
the IGM and primordial density fluctuation spectrum of some of the earliest
formed cosmological structures, as well as of the ultraviolet background
radiation that ionizes them.
