Revealing the Dark Threads of the Cosmic Web
  Modern cosmology predicts that matter in our Universe has assembled today
into a vast network of filamentary structures colloquially termed the Cosmic
Web. Because this matter is either electromagnetically invisible (i.e., dark)
or too diffuse to image in emission, tests of this cosmic web paradigm are
limited. Wide-field surveys do reveal web-like structures in the galaxy
distribution, but these luminous galaxies represent less than 10% of baryonic
matter. Statistics of absorption by the intergalactic medium (IGM) via
spectroscopy of distant quasars support the model yet have not conclusively
tied the diffuse IGM to the web. Here, we report on a new method inspired by
the Physarum polycephalum slime mold that is able to infer the density field of
the Cosmic Web from galaxy surveys. Applying our technique to galaxy and
absorption-line surveys of the local Universe, we demonstrate that the bulk of
the IGM indeed resides in the Cosmic Web. From the outskirts of Cosmic Web
filaments, at approximately the cosmic mean matter density (rho_m) and approx.
5 virial radii from nearby galaxies, we detect an increasing H I absorption
signature towards higher densities and the circumgalactic medium, to approx.
200 rho_m. However, the absorption is suppressed within the densest
environments, suggesting shock-heating and ionization deep within filaments
and/or feedback processes within galaxies.
