In the title, Heart of Darkness, the word "darkness" refers to the fact that the world is itself an evil thing, and civilized people have refused to accept that. People create morals to hide the truth that they really don't want to see. Hiding from what we really don't want to know is what humans do however, one day everybody who hasn't already noticed it will have to and will then realize that the world is not the safe and cozy place as once thought. As Marlow and Kurtz leave the inner station on the steam boat, they feel as though the have been able to leave all of the heartache of the jungle behind. However, as Marlow thinks to himself, "The brown current ran swiftly out of the heart of darkness, bearing us down towards the sea with twice the speed of our upward progress; and Kurtz's life was running swiftly, too, ebbing, ebbing out of his heart into the sea of inexorable time", (pg. 137) he realizes that the jungle is the heart of darkness and as their ship takes off and Kurtz is slowly dying, his compassion for the people will always be remembered. In the story, Heart of Darkness, "darkness" is referred to as the traditional symbol of evil, hatred, and fear.
Darkness can be viewed as the inability to see, and failing to see another human means failing to understand that person and failing to establish any sort of sympathetic unity with him or her. As Marlow thinks to himself, "It was unearthly, and the men were-No, they were not inhuman. Well, you know, that was the worst of it-the suspicion of their not being inhuman. It would come slowly to one. They howled and leaped, and spun, and made horrid faces; but what thrilled you was just the thought of their humanity-like yours-the thought of your remote kinship with this wild and passionate uproar", (pg. 100) it is made obvious to the reader that he looks down on Africans rather than think of them as equal to himself. Darkness is the obvious tone of the story and truly represents what many of the white people think of the African tribes, their members and their surroundings.
Darkness is also portrayed in the story through the manager of the Company when Kurtz is sick. His main concern instead of taking care of him, is to get himself some ivory. Darkness means very many different things and is portrayed through many different people however it remains the same that the entire point of darkness is how uncivil the Company is towards the African people.