  At first, a review of our knowledge on the distribution of galaxies at
large-scale, leading to a foam-like large-scale structure of the Universe, is
presented in the Introduction. Then, it is shown how, according to the present
theory for the formation of superclusters, wave scalar perturbations of the
same frequency traveling in opposite directions give rise to standing waves,
which cause a motion of the cosmic material towards the nodes, resulting in the
concentration of the cosmic material around the nodes. Generalizing this effect
to two (three) dimensions, the cosmic material is concentrated around the node
lines (node surfaces). It is proposed that the three-dimensional effect is
responsible for the foam-like large-scale structure of the Universe.
