The largest coherent structures that we can see in surveys are actually
superclusters that are much larger.
But the dynamical evidence indicates that they're still collapsing or forming.
Galaxy clusters are what we say virialized, which is their motions
are governed by the virial theorem, and so they are an equilibrium.
The galaxy clusters provide our best laboratory for
the fact that galaxies can change their properties over cosmic time.
In the centers of galaxies, we see the effect of galaxy interactions and mergers.
Usually, a dense galaxy cluster has a giant CD galaxy at its center,
an object with a trillion times the stellar mass of the sun.
Which is maybe ten or
twenty times more stellar mass than the Milky Way itself at the center.
These galaxies also tend to have intense x-ray emission from the hot gas they
contain or attract to them.
These are the largest galaxies in the universe, and
they've got that way by creating other galaxies of similar size, and
much larger number of dwarfs over a ten billion years.
Galaxy clusters are important laboratories for studying and
understanding dark matter.
It turns out that there are three completely independent ways to measure
the mass of a galaxy cluster.
The first is the first method used historically to infer dark matter.