Dark Matter Host

The Host is never in the picture. The picture is bent around them.

Weigh a galaxy cluster three ways. Count the light from its stars. Measure the X-rays from the hot gas between the galaxies, which outweighs the stars. Then measure how strongly the cluster bends the light of galaxies far behind it, using the second formula on this page, which turns a lensing angle into a mass. The third number is several times the first two put together. Fritz Zwicky saw a version of this in the 1930s from how fast the galaxies of the Coma cluster move. Vera Rubin and Kent Ford saw it in 1970 from how fast the outer parts of the Andromeda galaxy rotate, the first formula.

The sharpest picture came in 2006. The Bullet Cluster is two clusters that have passed through each other. Their hot gas collided, slowed and piled up in the middle, glowing in X-rays. Their galaxies, mostly empty space, sailed on through. Douglas Clowe and colleagues mapped the lensing and found the mass travelling with the galaxies, not with the gas, even though the gas holds most of the ordinary matter. Whatever carries most of the mass went through the collision almost without touching anything.

The cosmic microwave background says the same thing from the far end of time. The Planck satellite's 2018 results put about five times as much dark matter as ordinary matter in the universe: roughly 85 percent of all matter.

What it is remains open. No dark-matter particle has been detected directly, despite decades of searching. A minority of physicists argue for modifying gravity instead, though the Bullet Cluster is hard for most such theories to explain.

The Host never casts a shadow. They cast a weight.