  We introduce phenomenological understanding of the electromagnetic component
of the physical vacuum, the EM vacuum, as a basic medium for all masses of the
expanding Universe, and "Casimir polarization" of this medium arising in the
vicinity of any material object in the Universe as a result of conjugation of
the electric field components of the EM vacuum on both sides ("external" and
"internal") of atomic nucleus boundary of the each mass with vacuum. It is
shown that the gravitational attraction of two material objects in accordance
with Newton's law of gravity arises as a result of overlapping of the domains
of the EM vacuum Casimir polarization created by atomic nuclei of the objects,
taking into account the long-range gravitational influence of all masses of the
Universe on each nucleus of these objects (Mach's idea). Newton's law of
gravitational attraction between two bodies is generalized to the case of
gravitational interaction of a system of bodies when the center of mass of the
pair of bodies shifted relative to the center of mass of the system. The unique
smallness of gravitational interactions as compared with the fundamental
nuclear (strong and weak) and electromagnetic ones is determined by the ratio
of the characteristic size of the domain of EM vacuum Casimir polarization in
the vicinity of atomic nuclei to the Hubble radius of the Universe.
