Large-Scale Structure of Baryonic Matter: With its high FUV spectroscopic sensitivity, COS uniquely suited for exploring the Lyman-alpha forest. This is the ‘forest’ of absorption spectra seen in the spectra of distant galaxies and quasars caused by intergalactic gas clouds, which may contain the majority of baryonic matter in the universe. Because the most useful absorption lines for these observations are in the far ultraviolet and the sources are faint, a high sensitivity FUV spectrograph with wide wavelength coverage is needed to perform these observations. By determining the redshift and line width of the intervening absorbers, COS will be able to map out the temperature, density and composition of dark baryonic matter in the Cosmic Web.