X-ray Vision Reveals Intergalactic Medium

Invisible web. In this computer simulation, WHIM (blue) runs along filaments between galaxies and clusters of galaxies.

Three ingredients make up the universe. Normal atomic matter accounts for less than 5%, according to analysis of the microwave afterglow of the big bang. About 22% of the universe consists of mysterious dark matter--weighty stuff whose gravity appears to keep the stars from flying out of galaxies and to hold clusters of galaxies together but that has never been directly observed. A whopping 73% of the universe consists of bizarre space-stretching dark energy that's accelerating the expansion of the universe.