  The relativistic hydrodynamical equations are being examined with the aim of
extracting the quantum-mechanical equations (the relativistic Klein-Gordon
equation and the Schr\"odinger equation in the non-relativistic limit). In both
cases it is required to get the quantum potential, which follows from pressure
gradients within a superfluid vacuum medium. This special fluid, endowed with
viscosity allows to describe emergence of the flat orbital speeds of spiral
galaxies. The viscosity averaged on time vanishes, but its variance is
different from zero. It is a function fluctuating about zero. Therefore the
flattening is the result of the energy exchange of the torque with zero-point
fluctuations of the physical vacuum on the ultra-low frequencies.
