How dark are filaments in the cosmic web?
  The cold dark matter model predicts that dark matter haloes are connected by
filaments. Direct measurements of the masses and structure of these filaments
are difficult, but recently several studies have detected these
dark-matter-dominated filaments using weak lensing. Here we study the
efficiency of galaxy formation within the filaments by measuring their total
mass-to-light ratios and stellar mass fractions. Specifically, we stack pairs
of Luminous Red Galaxies (LRGs) with a typical separation on the sky of $8
h^{-1}$ Mpc. We stack background galaxy shapes around pairs to obtain mass maps
through weak lensing, and we stack galaxies from the Sloan Digital Sky Survey
(SDSS) to obtain maps of light and stellar mass. To isolate the signal from the
filament, we construct two matched catalogues of physical and non-physical
(projected) LRG pairs, with the same distributions of redshift and separation.
We then subtract the two stacked maps. Using LRG pair samples from the BOSS
survey at two different redshifts, we find that the evolution of the mass in
filament is consistent with the predictions from perturbation theory. The
filaments are not entirely dark: their mass-to-light ratios ($M/L = 351\pm137$
in solar units in the $r$-band) and stellar mass fractions ($M_{\rm stellar}/M
= 0.0073\pm0.0030$) are consistent with the cosmic values (and with their
redshift evolutions)
