The book is controversial or it has been made controversial by readers and critics.
The Heart of Darkness, is it about the African darkness. The darkness of the immense forests of the former Belgian colony Congo? Or is it the darkness of the Congolese soul, to which Joseph Conrad travelled? Or is it the darkness of the colonizers he met during his journeys?
Cinua Achebe wrote about this book. Barack Obama wrote about this book, and many others.
Here is another attempt to understand the book.