Insight into the meaning of the short poem "A Man Said to the Universe" by Stephen Crane can be derived by taking a look at the poet's life and work. Crane grew up in a religious household. His father was a Methodist minister and his mother was a devout practitioner of the faith. He picked up an interest in writing from his parents, who were both writers, and his brothers, who were journalists. In 1891, he dropped out of Syracuse University based on a decision that he could learn more directly from humanity than from his college curriculum.

His first novel was Maggie: A Girl of the Streets, which he self-published after traditional publishers rejected it because of its realistic depiction of the slums. His next novel, The Red Badge of Courage, a graphically depicted story set during the Civil War, made him famous.

Stephen Crane is considered one of the first practitioners of naturalism in American literature. Naturalism rejects romanticism and focuses on realism, environment, and social situations. In its biography and analysis of Stephen Crane and his work, the Poetry Foundation says this:

Crane viewed individuals as victims of purposeless forces and believed that they encountered only hostility in their relationships with other individuals, with society, with nature, and with God.

After considering this background, we can come back to the poem "A Man Said to the Universe." The man says that he exists. By doing this, in his own viewpoint, he is proclaiming his inherent importance and asserting that he has a role in the functioning and ultimate design of the universe.

The universe replies with indifference. The existence of the man means nothing to the universe one way or the other. Note that Crane does not have the man address his remark to God, a spiritual entity, but rather to the universe, which is a natural phenomenon. Through this poem, Crane is rejecting the religion of his youth and indicating that humankind is alone in a vast indifferent universe. Therefore, people must somehow solve their own problems of poverty, hatred, war, and so on, without the help of any divine power.

A man declares to the universe that he exists in order to be recognized, in order to feel that his existence is recognized, that he has meaning and purpose in the grand scheme of things. It is not enough for the man to conceive of his existence to and for himself. He is seeking recognition, or at least to be acknowledged by the universe around him. The universe replies that the fact that the man exists does not create a sense of the universe being obligated to that man. In other words, the (personified or sentient) universe acknowledges that the man exists but is indifferent to his existence. On the one hand, the universe gives the man what he wants; acknowledgment of his existence. But on the other hand, the universe essentially says, "yes, but it doesn't matter to me." 

Some readers might interpret this poem as a conversation with God, the universe being God. In this case, God acknowledges the man's existence but is not obligated to him. This would be a Deist perspective; a belief that God does not intervene in the world and in human affairs. The man is left to find the divine in himself and/or the abstract world of the spiritual. 

Reading this poem with "The Open Boat," it seems more likely that the universe is not God. Therefore, the poem is about man's quest for significance in a world that does not acknowledge him in a way that an intervening God would. The man is therefore faced with being acknowledged but ignored; just as the natural world reacts to him in physical ways but, having no consciousness, nature ignores him. In "The Open Boat," when the men are faced with drowning, the narrator (perhaps also the voice of the correspondent), notes: 

When it occurs to a man that nature does not regard him as important, and that she feels she would not maim the universe by disposing of him, he at first wishes to throw bricks at the temple, and he hates deeply the fact that there are no bricks and no temples. Any visible expression of nature would surely be pelleted with his jeers. 

In the poem, the man may have similar frustrations. He is in a world/universe which does not care about him. His only hope to feel significant is to accept the indifference of the universe to his fate and create significance for himself.