The title points to the cosmic microwave background, the earliest light we can detect, and the poem begins by saying we cannot see past it. The speaker tells a "you" that the beginning is closed off, and that we would sooner understand stones or the music of suns. Even our own dreams, with their hooded figure and revived rabbit, stay strange to us.
The third stanza turns personal. The speaker gathers the last words of a father, a sister and a great-aunt, which are odd, partial, and never quite explain anything. The great-aunt's "universe revealed" ties these small deaths to the large question of origins and endings.
The final stanza imagines the end of everything: stars out, black holes gone, every breath counted. The mood is awed and quietly sad, but not despairing. The last line, "Revealed," sounds both like a wonder and like a door closing, as if understanding arrives only when nothing is left to understand.