Holy moly this is badass. But I don't understand how gas 'tendrils' between galaxies accounts for half of the matter in the universe. Both teams took advantage of a phenomenon called the Sunyaev-Zel’dovich effect that occurs when light left over from the big bang passes through hot gas. As the light travels, some of it scatters off the electrons in the gas, leaving a dim patch in the cosmic microwave background – our snapshot of the remnants from the birth of the cosmos.
It's quite a bit of matter, but distributed over a huge amount of space. There's a lot of space in between galaxies, so the average density is nowhere near as high as a galaxy's, but it all adds up to be a large amount of the total mass in the universe. The Milky Way's diameter is 100,000 light years, and the distance to our nearest neighbor, Andromeda, is 2.5 million light years. I would suspect that the amount of matter in the tendrils connecting two galaxies is inversely proportional to the distance between them (but I don't care enough to actually read the paper, because my unfortunately finite intellectual capital is already stretched vanishingly thin at the moment).
Ah, duh, there's a lot of space in space. Thanks.