What does "the dark" symbolize in Camus' novel The Stranger?
One of the points of Camus' novel is that since there is no underlying, inherent meaning in life (though Camus disclaimed existentialism), meaning comes directly through sensory experience, and this sensory experience can completely overwhelm and in fact determine the course of events. While Camus does not advocate of Fate, he strongly suggests the determinism of sensory experience. Thus it is the light that Meursault blames for his actions on the beach.
We find this credible--though the judge and jury do not--because we (not they) have been with him from the time he received the message about his mother's death and have witnessed his struggles against the forces of the world. The dominant force he contends with is light and its accompanying heat (natural and artificial both). Light and heat have been presented paradoxically since they warm and comfort him, as when the light rests on his feet, and they agitate and discomfit him, as when he rides out to his mother's funeral, is at the wake and, most particularly, when he is at the beach.
I could see movements behind the windows; then all was calm again. The sun had risen a little higher and was beginning to warm my feet.
After having had my eyes closed, I had a feeling that the light had grown even stronger than before. There wasn’t a trace of shadow anywhere, and every object, each curve or angle, seemed to score its outline on one’s eyes.
The dark is the antithesis of light and it too has paradoxical elements. It sometimes sooths, as in sleep, though it also isolates and alarms, as in a solitary prison cell. Therefore, symbolically, the dark might be said to represent one of the inexplicable and meaningless elements that govern life's experience in a world devoid of ordered meaning where paradoxical experience is all that we have and yet is nothing that we can rely on.