The Universe remains cold until about a redshift of 2 in the simulation. At that point gas is falling into the gravitational "wells" generated by clumps of dark matter at speeds in excess of 200 km/s (not directly visible in the simulation). The gas being sucked into the regions of high dark matter density collides with itself, generating shock waves that heat the gas to millions of K and it begins to emit X-rays.

The highest temperatures, and thus the most intense X-ray emission, occurs at the intersection of filaments (the red knots in the display). Warm gas in the 100,000 - 1 million degree range defines the filaments (green in the display) but gas at this temperature would be almost invisible to X-ray telescopes.

This simulation uses a mixture of hot and cold dark matter. Here are further details of the calculation. Usage: Click "Play" to begin. Click "Stop" to pause. "Step" moves forward one frame, and "Back" moves backwards one frame. "Home" returns to the beginning.