Darkness makes a few cameo appearances in the drama of the speaker's crying-baby dilemma. It's not always just the absence of light, though. Throughout the poem and with some tricky line-breaks, the speaker acknowledges the dark as something with lifelike, human qualities (a neat trick called personification), which makes the darkness an integral part of the speaker's experience. It's a force to be reckoned with.
Line 4: The speaker ends the line with "his dark", meaning his son's dark bedroom. The enjambment puts an emphasis on the word "dark" in two ways. The first is that "dark" is a slant rhyme with the word "stars." It's a nice light-dark contrast, there, rhyming two words that are completely opposite. But the line break also makes the word dark work as a noun and an adjective. If we just read line 4, dark belongs to the son, as if it's a metaphorical darkness of his son's discomfort. But if when we read on to line 5, it's no longer "his dark", but "his dark / bedroom," so dark becomes an adjective for where they're standing. This play on words links the emotional with the physical. While they're standing in darkness, there's also an emotional darkness that's present, which surfaces as the son is breastfed and the speaker gets all jealous.
Line 9: The speaker begins this line with "In the dark" and goes on to say that "mothers illuminate like the stars!" Here again is a contrasting image of darkness and mothers lit up like stars. Of course, he's not saying mothers literally light up and glow, but he's using a simile to associate mothers with stars. That means she's praiseworthy, and she alone can take away the son's dark. After all, she's the only one that can feed him, and that darkness inside the boy is hungry!
Line 15: Here, darkness is written as "the hungry dark." This is an example of personification, where the dark takes on a human quality of hunger. Sounds sort of scary, right? Who wants to be swallowed by darkness? Probably nobody, especially not the speaker of this poem. He could be talking about the dark in figurative terms again, as something he has to feed, or that wants his attention, but is incapable of satisfying. Like a personal darkness, for example, some place inside that feels like it can never be satisfied.