Ok, Keith, the slight issue there is that because any material in filaments and nodes will be cold, it is very hard to detect. In clusters of galaxies it is hot and we do detect it. Where this statement comes from is simulations of the universe (such as the Millenium simulation, though as you can tell from its name its not the most recent project here). In the simulations run on computers, which reproduce very well the distribution of galaxies, though less well their properties, there is very little matter between the filaments. In the real universe it can't be measured because any such material will be cold and unobservable. If its molecular hydrogen especially.

So this statement may be right, but its not verified by observation. In clusters of galaxies we know there is lots of gas, although still at a lower density than in galaxies. But the volume is much higher, and the total mass is greater.