They are usually found close to the centers of galaxy clusters.

The number of galaxy clusters that form depends on dark energy in two ways.

The dark energy survey will discover a large number of galaxy clusters.

These may take the form of two quasars in the same galaxy cluster.

He has also advanced a new hypothesis on the structure of galaxy clusters.

It is the result of a still ongoing merger between two galaxy clusters.

It is also possible that the quasars are moving past each other within a cluster of galaxies.

They can be found near the centres of some rich galaxy clusters.

When one is found, others are surprisingly likely to be nearby; and they also tend to show up in clusters of galaxies.

It will help to probe the nature of dark energy by finding very distant galaxy clusters.