Close-up view of one filament in the EAGLE-Ref100 simulation. Shown is the dark matter (DM; top) and gas (bottom) surface density in a 6 × 6 × 6 Mpc cube at z = 0. In both cases, the colour map changes halfway between the cosmic average ($\overline{\mathrm{\Sigma }}$) and the 99.99 percentile to highlight the low- and high-overdensity regions. Galaxies with M_star > 10⁹ M_⊙ are depicted as open green circles. The DM is concentrated into clumps with a density > 10× above the smooth filament background. The gas is more smoothly distributed and shows a continuous filament structure, but still has noticeable peaks up to ≈5× the smooth background, due to the presence of galaxies and feedback-driven haloes. Neither corresponds to the smooth density field implicitly assumed by topological filament finders, which complicates the filament identification.