What is the central metaphor of the poem "Harlem" by Langston Hughes?

In the poem "Harlem," Langston Hughes creates a central metaphor surrounding a dream by comparing a dream to multiple images of death and destruction in order to ask what happens to a "dream deferred," meaning a dream that has been delayed in being fulfilled.

He first compares a delayed dream to a "raisin in the sun," implying the dream is dried up and shriveled. Next, he asks if a delayed dream becomes infected "like a sore." Untreated cuts become infected and painful, leading to other major health problems, sometimes even death; therefore, in comparing a delayed dream to an infected wound, Hughes likens a delayed dream to something that causes severe pain and is destructive, even deadly. A third comparison is of a delayed dream to "rotten meat," which is meat that has gone uneaten for so long it is now dangerous for anyone to consume. By comparing a delayed dream to inedible meat, Hughes is wondering if dreams that go unfulfilled for a long time become so painful that they are noxious to the dreamer.

A fourth comparison is that of a delayed dream to sugar or syrup, which implies the dreamer starts seeing the dream as having been sugarcoated, meaning more appealing than it actually is. An example of seeing a dream as sugarcoated could be an African American wishing for an end to segregation but being led to believe that sharing the world with white people would lead to even more abuses and inequalities. By being led to believe such a fallacy, an African American may believe his or her present situation is better than the situation he or she dreams of, which would make him or her see the dream as sugarcoated.

Two final comparisons are that of a delayed dream to a load that is too heavy to bear and to something that explodes. Hughes's final comparison of a delayed dream to an explosion is the most powerful because it significantly contrasts with his other images of death and destruction. By comparing a delayed dream to an explosion, he implies that when dreams are prevented from being fulfilled, the dreamer builds up enough energy until the dreamer explodes in a burst of energy to fulfill the dream, just as we saw with the dawning of the Civil Rights Movement.