  The cosmic dark ages and the epoch of reionization, during which the first
generations of stars and galaxies formed, are among the most compelling
frontiers of extragalactic astrophysics and cosmology. Here we describe an
exciting new probe of these eras: the 21 cm line of neutral hydrogen, which
will allow us to map the neutral intergalactic medium throughout the era of
first galaxy formation. In this white paper, we describe how these studies can
answer two key questions about galaxy formation: What were the properties of
high-redshift galaxies? How did they affect the Universe around them?
