The speaker is making a vow: to protect a bond with another person, which they imagine as a constellation. Stars that belong together are easy to scatter, so the speaker promises to keep them "in place" and arrange them again against the dark. The phrase "to fight against my whole body" is striking. It suggests that holding on takes real effort, as if the speaker's own fears or instincts might pull the connection apart.
The final lines carry the central feeling. The speaker calls the two of them "celestial secrets / the world will never be ready for." That reads as both proud and protective. The love, or closeness, is rare and cosmic, but it may also be something others wouldn't understand or accept. The mood is quiet, intimate and a little defiant. It feels like a whispered promise made in the dark, with the darkness serving as a canvas rather than something to fear.