The dark matter can be modeled as a scalar field using two fitted parameters, mass and self-interaction. In this picture the dark matter consists of an ultralight particle with a mass of ~10 eV when there is no self-interaction. If there is a self-interaction a wider mass range is allowed. The uncertainty in position of a particle is larger than its Compton wavelength (a particle with mass 10 eV has a Compton wavelength of 1.3 light years), and for some reasonable estimates of particle mass and density of dark matter there is no point talking about the individual particles' positions and momenta. By some dynamical measurements, we can deduce that the mass density of the dark matter is about 0.4 G e V c m − − 3 . One can calculate the average separation between these particles by deducing the de-Broglie wavelength: λ λ = 2 π π / m v , here m is the mass of the dark matter particle and v is the dispersion velocity of the halo. The average number of the particles in cubic volume having the dimension equal to the de Broglie wavelength, λ λ 3 is given by, N d b = ( 34 e V m ) 4 ( 250 k m / s v ) 3