Exploring the distribution of dark matter and dust in galaxy halos

Light rays from distant sources carry precious information about the matter along the line-of-sight. The well known example is the signature of intervening gas clouds through absorption lines imprinted in the spectra of background sources. In this talk I will show that the distribution of dark matter and dust around galaxies can also be revealed by analyzing the light from background quasars. Using the largest sample of QSOs available from the Sloan Digital Sky Survey, I will present new results on the magnification effects induced by galaxies and large-scales structures. I will show the corresponding mass distribution can be recovered on scales ranging from 50 kpc to 50 Mpc. I will then present the first detection of dust on large-scales around galaxies, obtained by measuring the reddening effects of background quasars. While such a result is of great interest for the study of galaxy evolution and outflows, it has also direct implications for dark energy measurements using supernovae as intergalactic dust is an additional source of extinction. I will discuss this new challenge and propose potential solutions for future dark energy experiments with upcoming surveys.