This cosmic web is not only poetic but important to the formation of galaxies and galaxy clusters. According to the Millennium Simulation, these filaments help to funnel gas into the "nodes" of the web, the sites of large galaxies and galaxy clusters. Our Milky Way Galaxy is thought to have formed in a small such node in this web.
Much of the material, over 80 percent, is dark matter, or matter that does not directly interact with electromagnetic radiation. Although we've detected its presence indirectly for decades due to its gravitational effects, we are no at the point where dark matter in the cosmos has been thoroughly mapped out. This gas, the "normal" matter that makes up people, stars, and planets, however, can be seen.
ANALYSIS: Scientists Discover Universe's Largest Structure
Since it is so diffuse, this gas proved to be a challenging target. It was found near a quasar in part because the quasar made it glow. It is also an ideal place for searching for such gas because quasars tend to inhabit the more densely populated nodes of the web, thus are likely to be surrounded by more of these filaments.
But, as always, there was a surprise in store. The mass of the gas cloud is ten times what has been predicted by simulations. A more massive cloud may have been an outlier, just the easiest to spot, but it is likely that the simulations don't quite model reality exactly. Some factors may be missing that would create gas clouds of this larger size as is seen in these recent observations. This just goes to show that even when you think you know what you are looking for, the Universe is going to surprise you.
This research was published in Nature; and a preprint is available at arXiv.org.