  Most of the baryons in the Universe are thought to be contained within
filaments of galaxies, but as yet, no single study has published the observed
properties of a large sample of known filaments to determine typical physical
characteristics such as temperature and electron density. This paper presents a
comprehensive large-scale search conducted for X-ray emission from a population
of 41 bona fide filaments of galaxies to determine their X-ray flux and
electron density. The sample is generated from Pimbblet et al.'s (2004)
filament catalogue, which is in turn sourced from the 2 degree Field Galaxy
Redshift Survey (2dFGRS). Since the filaments are expected to be very faint and
of very low density, we used stacked ROSAT All-Sky Survey data. We detect a net
surface brightness from our sample of filaments of (1.6 +/- 0.1) x 10^{-14} erg
cm^{-2} s^{-1} arcmin^{-2} in the 0.9-1.3 keV energy band for 1 keV plasma,
which implies an electron density of n_{e} = (4.7 +/- 0.2) x 10^{-4}
h_{100}^{1/2} cm^{-3}. Finally, we examine if a filament's membership to a
supercluster leads to an enhanced electron density as reported by Kull &
Bohringer (1999). We suggest it remains unclear if supercluster membership
causes such an enhancement.
