The speaker keeps returning to one word: empty. It opens the poem, it stands alone on its own line, and it comes back again, as if the feeling can't be argued with or explained. Nothing is added to soften it.
In the middle, the speaker looks up. A grey sky gives way to a weak purple light, and they notice something "small": distant stars, far apart. That image suggests loneliness, with each point of light separated from the others by huge gaps. Even the sky seems to mirror the speaker's isolation rather than comfort them.
The ending turns inward. The walls "will not look back," and the speaker says vision is "just a trick of the light," and so are they. It's a quietly unsettling thought: that they feel unreal, barely there, something made of light and easily gone. The mood is hushed, numb and lonely, closer to fading away than to dramatic sadness.