Well, by now you know that Larkin's "Aubade" doesn't fall into the feel-good poetry category. It's pretty dark. So, it isn't too surprising that the poem relies on dark imagery to set the mood. Larkin also sets up an extended metaphor: night and darkness represent death throughout the poem.
Line 1: The end word of the poem's first line is "night." The placement is significant. The line begins with day and ends with night. We are left for a moment to consider that darkness. Not only does the word "dark" come at the end of the first line, there is a full stop (a period) at the end of the line. That forces us to spend a little more time in the dark.
Line 2: Just in case we didn't get enough darkness hanging out with "night" in line 1, Larkin describes the "soundless dark" of pre-dawn in line 2. Describing the dark as "soundless" makes it feel kind of void-like. We know that we can't see in the dark, but this dark is also so silent that it seems soundless. It's as if the night is so dark that no stimulus can penetrate it.
Line 5: It doesn't take Larkin very long to get to death. Line 5 starts with "unresting death." See the progression? "Night" in line 1, "dark" in line 2, and "death" in line 5. The gang's all here!
Lines 6-10: Things are escalating here in stanza 1. We started with night, moved onto darkness, then to death in a kind of general sense, and finally we arrive at the very personal and frightening notion of our own imminent demise. The night and the darkness seem to have triggered the speaker's thoughts of death and dying.
Lines 17-20: Things just keep getting bleaker in stanza 2. That "sure extinction" the speaker talks about is none other that Larkin's ol' buddy, death. Larkin describes the experience of death as, "Not to be here, Not to be anywhere." Does that description remind you of anything from stanza 1? How about that void-like sightless and "soundless dark" description of night? Keep that connection between night-dark-death in mind—Larkin is just getting warmed up.
Lines 27-30: Larkin expands on the description of death's void: "No sight, no sound, no touch or taste or smell." Again, this description of death echoes that "soundless dark" description of night from stanza 1.
Line 37-40: Yup. How about some more death? This time Larkin makes sure to burst that last bubble of hope we might have been holding onto: be brave and face it, accept death and live life. Larkin thinks that's fine, but he wants you to know it doesn't matter. Death is the same for the brave and the cowardly alike.