  We find clusters and superclusters of galaxies using the Data Release 1 of
the Sloan Digital Sky Survey. Density fields with smoothing lengthst of 10 and
0.8 Mpc/h are calculated. We found that galaxies and clusters in a high-density
environment have luminosities that are five to ten times higher than in a
low-density environment. Numerical simulations show that in underdense regions
most particles form a rarefied population of pregalactic matter whereas in
overdense regions most particles form a clustered population in rich clusters.
Simulations show also that very massive superclusters are great attractors and
have small bulk motions.
