Cited research: Astrophys. J. 714, 1715–1724 (2010)

Peering into the local reaches of the Universe, astronomers find that they can see only about half the amount of atomic matter expected to be there. The other half is probably tied up in thin, wispy tendrils of gas called the warm-hot intergalactic medium (WHIM), say Taotao Fang of the University of California, Irvine, and his colleagues.

The researchers used two X-ray telescopes to look at a strong spotlight emanating from a blazar, a supermassive black hole at the centre of a distant galaxy. The light passed though a closer feature known as the Sculptor Wall, an agglomeration of galaxies also thought to contain a lot of WHIM. This 'wall' absorbed certain wavelengths of the light, allowing the researchers to confirm, with high precision, the presence of WHIM, which has a density 30 times that of the Universe's average. E.H.