Light is the main character here, and the poem follows it as it changes. At first it is tense and stiff. It meets darkness at "the edge of form" but can't make sense of it, and it falls back, "out of phase." The repeated opening line works like a refrain, and each time light does a little more. It flexes, bends, and begins to gather black holes into its "stretching spectrum."
The big turn comes when light stops resisting the dark and makes room for it. Darkness becomes part of the music, an overtone inside the song of radiance. By the end, light "breathes dark voids" and "exhales galactic chords," and the void is even induced to dance.
The speaker sounds more like an awed observer than someone in distress, and the mood moves from strain to openness and joy. The poem seems to suggest that growth comes from absorbing what opposes you, not defeating it. The mix of physics and music makes that idea feel cosmic but also warm.