The presence of neutral hydrogen in this region causes a suppression of wavelengths less than the Lyman-alpha emission line in quasar spectra; that feature, which helps determine the time at which this substance reionized, is called the Gunn-Peterson trough.  
One piece of evidence for the existence of this feature was found by Jakobsen et al, who found a dropoff in the UV spectrum of Q0302-003. Rorai, Hennawi, and White found a method to measure the Jeans scale of this region using quasar pairs. Somewhere under 75% of the gamma ray background of the universe is purportedly caused by shockwaves hitting this region during structure formation. Tracers for this region include oxygen-7 and oxygen-8 absorption lines. This region's ionization state is maintained by a diffuse background of UV photons. Cen and Ostriker claim that its warm-hot phase may contain between 30-50% of the (*) baryonic matter in the universe. The presence of neutral hydrogen in this region causes a suppression of wavelengths less than the Lyman-alpha emission line in quasar spectra; that feature, which helps determine the time at which this substance reionized, is called the Gunn-Peterson trough.  For 10 points, name this filamentous web of gas and plasma that occupies the space between galaxies.
One piece of evidence for the existence of this feature was found by Jakobsen et al, who found a dropoff in the UV spectrum of Q0302-003.
intergalactic medium [or IGM]