In "Macbeth", why does Shakespeare match bad events with dark imagery and bad events?It adds to the dramatic effect, but how can I expand on that explanation?
The better literary term for "dark imagery" is "pathetic fallacy" - which is when a writer uses the weather symbolically to demonstrate some point about the human world. And, of course, this is exactly what Shakespeare is doing in "Macbeth": adding to the drama, yes, but also creating the sense that Macbeth's crime (killing the reigning monarch, Duncan) has not just upset the mortal world, but also created a "breach in nature" itself.
Thou seest the heavens, as troubled with man's act,
Threaten his bloody stage.
(Quote is from 2, 2 - although the enotes link claims to be to 4, 4)
So how do you expand on it? Pathetic fallacy does not only create a dramatic effect, but it implies cosmic (maybe even godly - depending on how you read the play) consequences upon the human world.
Dark imagery is a universal symbol that something bad is happening or about to happen. Think of a horror movie. It's full of dark images, nighttime, storms, dilapidated houses, things that set the mood of the story. Shakespeare did the same thing. He wants the audience to understand that the mood occurring on stage is a signal for something ominous that either just occurred, is occurring on stage, or is about to occur. Dark imagery is a universal warning that audience recognize immediately so they are prepared or expect something bad or scary to occur.