  Cosmological simulations predict that a large fraction of the baryonic mass
of the Universe exists as 10^5-10^7 K diffuse, X-ray emitting gas, tracing low
density filament and sheet-like structures exterior to massive clusters of
galaxies. If present, this gas helps reconcile the current shortfall in
observed baryon counts relative to the predictions of the standard Big Bang
model. We present here the discovery and analysis of a 5-sigma significance
half-degree filamentary structure, present in both I-band galaxy surface
density and unresolved X-ray emission in a deep ROSAT PSPC field. The estimated
diffuse X-ray emission component of this structure has a surface brightness of
\~ 1.6 x 10^(-16) erg/s/cm^2/arcmin^2 (0.5- 2 keV), comparable to the
predictions for inter-cluster gas and may represent a direct detection of this
currently unconfirmed baryonic component.
