  We discuss how measurements of the absorption of gamma-rays from GeV to TeV
energies via pair production on the extragalactic background light (EBL) can
probe important issues in galaxy formation. Semi-analytic models (SAMs) of
galaxy formation, based on the flat LCDM hierarchical structure formation
scenario, are used to make predictions of the EBL from 0.1 to 1000 microns.
SAMs incorporate simplified physical treatments of the key processes of galaxy
formation -- including gravitational collapse and merging of dark matter halos,
gas cooling and dissipation, star formation, supernova feedback and metal
production. We will summarize SAM successes and failures in accounting for
observations at low and high redshift. New ground- and space-based gamma ray
telescopes will help to determine the EBL, and also help to explain its origin
by constraining some of the most uncertain features of galaxy formation theory,
including the stellar initial mass function, the history of star formation, and
the reprocessing of light by dust. On a separate topic concerning gamma ray
cosmology, we discuss a new theoretical insight into the distribution of dark
matter at the center of the Milky Way, and its implications concerning the high
energy gamma rays observed from the Galactic center.
