The circumgalactic medium contains signatures of key processes in galaxy formation, such as gas accretion and outflow, and may account for the majority of baryons in the Universe. To probe gas in this environment, I developed new methods to model quasar spectra and measure absorption induced by gas in galaxy halos. Applying the new tools to all quasar spectra in the SDSS, I have compiled a metal absorber catalog of ~50,000 systems and, for the first time, measured the large-scale distribution of gas from galaxies out to ~20 Mpc, linking the gas properties of individual galaxies to their large-scale environment. I will further discuss the new constraints of these results bring to the physics of circumgalactic medium and its role in galaxy formation and evolution in general.