What is the significance of the book title HEART OF DARKNESS by Joseph Conrad?

The title has multiple meanings. Think about Marlow's journey from what he perceives as civilization into the wilds of an unfamiliar land. It also can refer to the darkness within-- within Kurtz, obviously, or even within Marlow himself. Or you can apply it to the encomping theme of imperialism, which masquerades on being beneficial to those conquered while often hiding less benevolent goals.