Matter is either substance or electromagnetic waves. A shadow is neither of them. The shadow is a pattern formed when light is blocked by an object and perhipheral light to the sides of the object strikes the surface. In other words the shadow represents a zone where photons (light waves) emanating from a source are blocked by impeding matter. Thus, it is an optical phenomenon but it is not made up of any matter. Light is matter, impending object is matter, but shadow is not. The shadow is caused by matter but is not matter itself.