Only in recent years has the realization emerged that galaxies do not dominate the universal baryon budget but are merely the brightest pearls of an underlying Cosmic Web .
Although the gas in these inter-galactic filaments is moderately to highly ionized , QSO absorption lines have shown that the surface area increases dramatically in going down to lower HI column densities .
The first image of the Cosmic Web in HI emission has just been made of the Local Group filament connecting M31 and M33 .
The corresponding HI distribution function is in very good agreement with that of the QSO absorption lines , confirming the 30-fold increase in surface area expected between 10 ^ { 19 } cm ^ { -2 } and 10 ^ { 17 } cm ^ { -2 } .
The critical observational challenge is crossing the “ HI desert ” , the range of log ( N _ { HI } ) from about 19.5 down to 18 , over which photo-ionization by the intergalactic radiation field produces an exponential decline in the neutral fraction from essentially unity down to a few percent .
Nature is kinder again to the HI observer below log ( N _ { HI } ) = 18 , where the neutral fraction decreases only very slowly with log ( N _ { HI } ) .
With the SKA we can begin the systematic study of the Cosmic Web beyond the Local Group .
With moderate integration times , the necessary resolution and sensitivity can be achieved out to distances beyond the Virgo cluster .
When combined with targeted optical and UV absorption line observations , the total baryonic masses and enrichment histories of the Cosmic Web will be determined over the complete range of environmental over-densities .