why dark ?

Ruprecht von Kaufmann

....because the brighter a light shines, the more darkness it reveals

Unfortunately there can be no doubt that man is, on the whole, less good than he imagines himself or wants to be. Everyone carries a shadow, and the less it is embodied in the individual's conscious life, the blacker and denser it is. If an inferiority is conscious, one always has a chance to correct it. Furthermore, it is constantly in contact with other interests, so that it is continually subjected to modifications. But if it is repressed and isolated from consciousness, it never gets corrected.
"Psychology and Religion" (1938). In CW 11: Psychology and Religion: West and East. P.131

How else could it have occurred to man to divide the cosmos, on the analogy of day and night, summer and winter, into a bright day-world and a dark night-world peopled with fabulous monsters, unless he had the prototype of such a division in himself, in the polarity between the conscious and the invisible and unknowable unconscious? Primitive man's perception of objects is conditioned only partly by the objective behaviour of the things themselves, whereas a much greater part is often played by intrapsychic facts which are not related to the external objects except by way of projection. This is due to the simple fact that the primitive has not yet experienced that ascetic discipline of mind known to us as the critique of knowledge. To him the world is a more or less fluid phenomenon within the stream of his own fantasy, where subject and object are undifferentiated and in a state of mutual interpenetration.
"Psychological Aspects of the Mother Archetype" (1939) In CW 9, Part 1: The Archetypes and the Collective Unconscious. P. 187

The hero's main feat is to overcome the monster of darkness: it is the long-hoped-for and expected triumph of consciousness over the unconscious. The coming of consciousness was probably the most tremendous experience of primeval times, for with it a world came into being whose existence no one had suspected before. "And God said, 'Let there be light"' is the projection of that immemorial experience of the separation of consciousness from the unconscious.
"The Psychology of the Child Archetype" (1940). In CW 9, Part I: The Archetypes and the Collective Unconscious. P.284

If you imagine someone who is brave enough to withdraw all his projections, then you get an individual who is conscious of a pretty thick shadow. Such a man has saddled himself with new problems and conflicts. He has become a serious problem to himself, as he is now unable to say that they do this or that, they are wrong, and they must be fought against. He lives in the "House of the Gathering." Such a man knows that whatever is wrong in the world is in himself, and if he only learns to deal with his own shadow he has done something real for the world. He has succeeded in shouldering at least an infinitesimal part of the gigantic, unsolved social problems of our day.
"Psychology and Religion" (1938). In CW 11: Psychology and Religion: West and East. P.140

Taking it in its deepest sense, the shadow is the invisible saurian tail that man still drags behind him. Carefully amputated, it becomes the healing serpent of the mysteries. Only monkeys parade with it.
The Integration of the Personality. (1939).

The Shadow