  One of the next decade's most exciting prospects is to explore the cosmic
"dark ages," during which the first stars in the Universe formed, with the 21
cm line of neutral hydrogen. At z>6, this light redshifts into the
low-frequency radio regime and can be observed with new generations of
wide-field arrays. These experiments have the potential to observe enormous
volumes and to improve both cosmological and astrophysical constraints
dramatically. In this white paper, we describe how the next decade will see the
first steps toward unlocking this potential, allowing us to answer two key
questions about the fundamental physics of our Universe and the intergalactic
medium: Does the standard cosmological model describe the Universe during the
"dark ages?" How did the intergalactic medium evolve during this important
time, ending with the reionization of hydrogen?
