Galaxy Clusters
Galaxy clusters are formations of Dark Matter, galaxies and hot gas in the knots of the cosmic web. They are as heavy as a thousand Milkyways, and consist of about 80% Dark matter, 15% normal matter, and less than 5% of stars bound in galaxies.
A composite image of the merging galaxy cluster 1E 0657−55.8 , named the "Bullet cluster". Red: Hot cluster gas. Blue: Dark Matter distribution. Background: Optical light with hundreds of galaxies. Clowe and colleagues for NASA
The gas in their center is very hot, about as hot as the center as our sun. It emits X-ray radiation, which is observed with satellites like the Chandra observatory. Physicists call this state of matter a plasma. In fact, it is the most perfect plasma we know ! This makes the medium a unique laboratory for plasma physics.
Clusters are so large that despite their weight, there would be only very few hot particles in a liter bottle “full” of cluster gas. As the particles are flying through the gigantic volume, they nearly never bump into each other, particle collisions are very rare. Instead, they sense each others presence through a version of light we call plasma waves, like a boat that is rocked in a stormy sea.
The interplay of this mechanism with other components (magnetic fields, high speed particles, atoms, molecules) makes the physics of the medium incredibly complex and interesting to study.