  (abridged) We present the results of a redshift survey of intercluster
galaxies in the central region of the Shapley Concentration supercluster, aimed
at determining the distribution of galaxies in between obvious overdensities.
Our sample is formed by 442 new redshifts, mainly in the b_J magnitude range
17-18.8. Adding the data from our redshift surveys on the A3558 and A3528
complexes, which are close to the geometrical centre of this supercluster, we
obtain a total sample of ~2000 radial velocities. Using the 1440 galaxies of
our total sample in the magnitude range 17 - 18.8, we reconstructed the density
profile in the central part of the Shapley Concentration; moreover we detected
another significant overdensity at ~30000 km/s (dubbed S300). We estimate the
total overdensity in galaxies, the mass and the dynamical state of these
structures, and discuss the effect of considering a bias between the galaxy
distribution and the underlying matter. We find an indication that the value of
the bias between clusters and galaxies in the Shapley Concentration is higher
that that reported in literature, confirming the impression that this
supercluster is very rich in clusters. Finally, from the comparison with some
theoretical scenarios, we find that the existence of the Shapley Concentration
is more consistent with the predictions of the models with a matter density
parameter <1, such as open CDM and Lambda CDM.
