In the same way different objects have different weights according to the area they cover. The source of the light that reach on the earth are mainly 3 the one is the sun the second one is the moon and the third one which is visible in the sky and is the bright is venus. It is also the bright source of light. The light that is comming from the venus is highest than the sun as the light from the sun requires the medium while the venus dont. It directly propagate through the vacuum. Hence the light from the vacuum gives more weight than the others does.
Now we may also have the question if the shadow made up of atoms as it is claimed that every matter in this universe are made up of atoms but as the shadow is the objection of the light blockage so we cannot say or hope that they contian the atom. The light consists of photon that is the particle but having no weight. Thus in this sense we can say that it is without atoms and that actually carries the weight by the particles that they collide and it depends how light is passing. If the light is heavy then dark blck shadow is form in the same way if the light is fade then the weak shadow is form.