Why is there darkness in humanity?

Darkness can be viewed in a literal form and/or in an emotional and implicational form.  Literally, darkness is an essential part of life that has to occur in order for everyone to have a full day. However, if we look at it like a connotation it is still just as essential in life as it is literally. Darkness/evil, happens,  I believe because we let it happen. But, if we didn't let it happen we could possibly be living in a stereotypical, 'perfect world' which from experience that we have learned that it cannot exist, and darkness will eventually come in, and over power whatever eutopia like life we had. It will always be a part of our lives, so why do we question it.