The distribution of ordinary ("baryonic") matter, in the form of gas and dust grains, in the extended halos of galaxies has been largely unconstrained in recent decades, and thus subject to considerable debate. This matter is expected to carry signatures of key processes in galaxy evolution. But very recently, the Principal Investigator and collaborators have demonstrated that statistical reddening measurements can dramatically increase the sensitivity with which reddening effects by dust grains can be detected, and can be used to map out the presence of dust on scales ranging from roughly 20 kiloparsecs to several megaparsecs. This opens a new avenue for exploring the distribution of baryons around galaxies and understanding the enrichment of the intergalactic medium. In this project, the proposing team will extend their existing techniques in order to probe the distribution of dust around galaxies of different types, luminosities, redshifts, and environments. The planned improvements will provide unique information on feedback processes and galaxy evolution, and will provide new constraints on the opacity of the Universe as a function of redshift. They will also apply their techniques to the statistical detection of extremely weak absorption lines. The goal is to reveal the large-scale distribution of heavy elements in the gas phase, probing the physics of galaxy formation and its related feedback processes. The project will contribute to workforce development by supporting a postdoctoral researcher and a graduate student. The Principal Investigator, postdoc, and student will continue development of the STOMP library: a generic, public platform for doing fast spatial statistics using tens of millions of points. Results understandable to non-specialists will be incorporated in classroom lectures. In collaboration with the Johns Hopkins Center for Educational Outreach, the Principal Investigator will also give public talks in high schools of the Baltimore area.