Large-scale mass distribution in the Illustris simulation
  Observations at low redshifts thus far fail to account for all of the baryons
expected in the Universe according to cosmological constraints. A large
fraction of the baryons presumably resides in a thin and warm-hot medium
between the galaxies, where they are difficult to observe due to their low
densities and high temperatures. Cosmological simulations of structure
formation can be used to verify this picture and provide quantitative
predictions for the distribution of mass in different large-scale structure
components. Here we study the distribution of baryons and dark matter at
different epochs using data from the Illustris simulation. We identify regions
of different dark matter density with the primary constituents of large-scale
structure, allowing us to measure mass and volume of haloes, filaments and
voids. At redshift zero, we find that 49 % of the dark matter and 23 % of the
baryons are within haloes more massive than the resolution limit of $2\times
10^8$ M$_\odot$. The filaments of the cosmic web host a further 45 % of the
dark matter and 46 % of the baryons. The remaining 31 % of the baryons reside
in voids. The majority of these baryons have been transported there through
active galactic nuclei feedback. We note that the feedback model of Illustris
is too strong for heavy haloes, therefore it is likely that we are
overestimating this amount. Categorizing the baryons according to their density
and temperature, we find that 17.8 % of them are in a condensed state, 21.6 %
are present as cold, diffuse gas, and 53.9 % are found in the state of a
warm-hot intergalactic medium.
