2025NatAs...9..577T

Filaments connecting haloes are a long-standing prediction of cold-dark-matter theories. Here we present a detection of the cosmic web emission connecting two quasar-host galaxies at redshift z ≈ 3.22 in the MUSE Ultra Deep Field (MUDF), observed with the Multi Unit Spectroscopic Explorer (MUSE) instrument. The very deep observations unlock a high-definition view of the filament morphology, a measure of the transition radius between the intergalactic and circumgalactic medium, and the characterization of the surface brightness profiles along the filament and in the transverse direction. Through systematic comparisons with simulations, we validate the filaments' typical density predicted in the current cold-dark-matter model. Our analysis of the MUDF, an excellent laboratory for quantitatively studying filaments in emission, opens a new avenue to constrain the physical properties of the cosmic web and to trace the distribution of dark matter on large scales.