While there is no single factor in astrology that signifies the shadow, it has much in common with Pluto. As lord of the underworld, Pluto is naturally associated with things dark & shadowy. Pluto symbolizes the wounded-healer, which necessarily has two poles: wounded & healer. The dark, projected side appears to be wounded, while the light, identified side feels imbued with healing potential.
Workshop
NORWAC 2002