Galaxy clusters are considered the largest array of objects that are grouped together by gravity. They are the densest part of the large-scale structure of the universe.
In the photo above, there are over 4000 galaxies shown, called Abell, after the scientist who discovered them in 1958, George O. Abell. Some are hundreds of millions of light years apart from each other. Considering this is an image of clusters of galaxies, hundreds in each cluster, and when a typical galaxy contains between 100 – 200 billion stars with average diameters of 100,000 light years each, this is quite mind blowing.
Our Milky Way Galaxy is part of a cluster called the Local Group or Local Cluster and consists of over 54 galaxies. All clusters have a gravitational center and ours is located between the Milky Way and the Andromeda galaxy. 54 galaxies may sound extraordinary, but that is minuscule when considering that there are over 100 billion galaxies in the known universe.
For galaxy clusters, we measure their distances in units of parsecs. A distance of one million parsecs is commonly denoted by megaparsec (Mpc).
A parsec represents 1 degree of the angle that is created for 1 AU. If we draw a line of 1 AU (which is the distance between Earth and the Sun), then draw a line from each point (E and S), the point at which the two lines meet (D), we create a triangle of 1 arc second, so the distance between the Earth point and the point at which the lines meet is 1 parsec.
Dimensions not in scale.
1 Parsec is equal to the distance D from the Earth E by an angle of 1 arc second, which equates to 206,264.81 astronomical units (AU).
1 parsec = 206,264.81 astronomical units (AU) = 3.26 light years ~ 19 trillion miles.
There is an easy conversion utility for AU and parsecs here.
Our Local Group (of galaxies) has a diameter of 3.1 megaparsecs, which is, 3.1 x 1,000,000 parsecs in length or 3.1 mp * 3.26 ly.