Actually my question comes from Gauss law which says charges outside a closed surface, doesn't effect on the whole electric flux of the surface.
The reason of that is that the solid angle of a closed surface while measuring it from outside of the surface, is zero. ( on the other hand, the solid angle of a closed surface from inside the closed surface is $4\pi$)
My question is why solid angle of a closed surface from exterior is zero? Please don't use Stokes' theorem!