Re: What is the shape of the Virgo Supercluster ?

As a general rule, as you go up in scale in the universe, dynamical systems tend to become less flattened and more blobby. The reason for this is that if you have a bunch of masses which start out in some blobby configuration, gravity acts over time to flatten their distribution, and conservation of angular momentum will usually lead to even more flattening, such as we see in the solar system or in spiral galaxies.

Going up in scale to the Local Group: since that is dominated by just two galaxies, namely the Milky Way and M31, it's structure is more like a dumbbell. In that sense it is flattened, with most of the smaller galaxies spread in a rather fuzzy way around these two big guys.

The Virgo cluster probably is somewhat flattened (although it is hard to say for sure since we still only have approximate distances to many of its galaxies), but retains a good deal of blobbiness, in part due to the fact that it continues to grow with new groups of galaxies falling in every now and then, which stirs things up. A good way of seeing the shapes of galaxy clusters is to look at images taken in X-rays, which shows up the large clouds of hot plasma through which the galaxies are moving (this hot plasma contains more matter than the galaxies in fact). In the case of Virgo this clearly shows that the main cluster is around the galaxy M87, but that significant subgroups of galaxies are associated with M86 and M49.

Up in scale again, if one looks at the galaxies within about 200 million light years (which is about 4 times the Virgo distance), you find that although this contains many galaxy clusters, quite a few of them do seem to map out a roughly flattened structure. This is known as the supergalactic plane. It is important to realise that by this scale, gravity has not yet had chance to overcome the expansion of the universe. So the supergalactic plane as a structure is still expanding, even though the individual groups and clusters of galaxies that make it up are not.

The very largest scales on which we see any kind of flattening are less than about a billion light years. Redshift surveys, such as the 2dF survey show that the very largest sheets and filaments of galaxies approach this size. By this point the picture of the galaxy distribution is very much like a sponge, and the flatter structures are more a result of our eye picking out linear shapes in a tangled pattern of bubbles and sheets than they are of genuine dynamical entities.