The closest large cluster to us is a moderately rich one called the Virgo Cluster because it is in the direction of the Virgo constellation. It has about 2500 galaxies (mostly spirals and irregulars) distributed into an irregular shape about 3 million parsecs (or 3 Megaparsecs or Mpc) across. It is about 15-17 Mpc from us. Some ellipticals are present in the central part of the cluster including a giant elliptical at the center (M87) that has become so large by gobbling up nearby galaxies that were attracted by its enormous gravity. The total mass of the Virgo cluster is large enough that its gravity pulls nearby groups of galaxies (including the Local Group) toward it.

Here are two pictures of the Virgo Cluster:
  1. A large part of the Virgo Cluster
  2. A small part of the Virgo Cluster

The clustering phenomenon doesn't stop with galaxies. Galaxy groups and clusters attract each other to produce superclusters of over 10,000 galaxies each. Their mutual gravity binds them together into long filaments 100 to 300 Mpc long, 50 to 100 Mpc wide, and 5 to 10 Mpc thick on average. Between the filamentary superclusters are HUGE voids with very few (if any) galaxies.   This characteristic appearance is called the Large Scale Structure of the Universe.

 
The graphics below show two views of the filamentary distribution of galaxies on these very large scales.  Remember, each elongated structure you see is a supercluster containing many clusters and groups of galaxies.

The first plot shows the distribution on the sky of galaxies with a large range of distances.  It is oriented such that the disk of our Milky Way runs horizontally across the middle.  The second plot is a very different kind of view.  The Milky Way is at the center, and the plot shows distances to galaxies from a large study of two specially chosen "pie-slices" or wedges of the universe.  The lack of galaxies at the very top and bottom of the plot is not real but shows how hard it is to find galaxies so far from us because of their faintness.  The furthest ones are 250 Mpc from the Milky Way!  The fact that only these restricted directions in the sky have been studied in this way shows how large the undertaking is to fully map out the structure of the universe.

Note how the galaxies and clusters form shells or walls around relatively empty voids.   In recent years these surveys have been extended to reveal more of the three-dimensional structure, and an animation of the resulting distribution of galaxies and clusters for a pie-slice-shaped wedge of the universe can be viewed here.  The Milky Way is at the bottom of the plot.  The furthest galaxies are now 600 Mpc away!