  Gamma Ray Burst (GRB) afterglows close to their peak intensity are among the
brightest X-ray sources in the sky. Despite their fast power-law like decay,
when fluxes are integrated from minutes up to hours after the GRB event, the
corresponding number counts (logN-logF relation) far exceeds that of any other
high redshift (z>0.5) source, the flux of which is integrated over the same
time interval. We discuss how to use X-ray afterglows of GRBs as distant
beacons to probe the warm (10^5 K<T<10^7 K) intergalactic matter in filaments
and outskirts of clusters of galaxies by means of absorption features, the
``X-ray forest''. According to current cosmological scenarios this matter may
comprise 30-40% of the baryons in the Universe at z<1. Present-generation X-ray
spectrometers such as those on Chandra and XMM-Newton can detect it along most
GRBs' lines of sight, provided afterglows are observed fast enough (within
hours) after the burst. A dedicated medium-sized X-ray telescope (effective
area <=0.1 m^2) with pointing capabilities similar to that of Swift (minutes)
and high spectral resolution (E/DeltaE>=300) would be very well suited to
exploit the new diagnostic and study the physical conditions in the Universe at
the critical moment when structure is being formed.
