Hi, Raoul.

Overall this is a great website full of basic information (for a Novice like me).

Something else that may relate to 'our' Great Attractor:

Coma Cluster
Why Study the Coma Cluster?

The Coma Cluster is an ideal subject for observation. Less than five percent of Abell clusters contain as many galaxies as Coma does, and none are as close to the Earth. Because Coma is not obscured by the dust clouds of our galaxy, the Milky Way, it can be seen without interference. Since the advent of x-ray imaging, several phenomena have been observed in galaxy clusters and continue to be studied in Coma and elsewhere.

Missing Mass

Before x-ray astronomy, the amount of mass that could be observed in a cluster was based solely on visible data appearing as galaxies.

High-temperature gas clouds, which are detected using x-ray telescopes, fill the cluster. These clouds have as much mass as all the stars in all the galaxies in the cluster! The cluster also contains a large amount of mysterious dark matter, about ten times the amount in gas and galaxies. This matter has yet to be observed with any type of telescope. The gravitational pull of this dark matter in clusters holds the movement of member galaxies and gas in the cluster.

By mapping the x-ray emissions of Coma's gas cloud, the distribution of the gas can be modeled and the mass of the dark matter can be estimated more accurately. With this information from Coma and other clusters, the mass density of the entire universe could be calculated. This would allow astronomers to predict whether or not the universe will continue to expand indefinitely or if it will eventually begin to contract.

http://chandra.harvard.edu/xray_sources/coma/

The reason I posted Chandra's Coma Cluster site is because that may also be the best explanation for 'our' neighborhood's Great Attractor as well as your statement:

Galaxies seem to regroup in sort of islands in the middle cosmic rivers.

At least we can come to some conclusions based on what we know(even if we don't see it) and actually 'see'.

Cheers,

Marcia