Four short lines announce something enormous: our galaxy is speeding up and will probably crash into others. The news sounds alarming, but the poem delivers it in a flat, matter-of-fact voice, as if reading a weather report.
The humor, and the unease, come from the timing. "In two billion years" is so far off that no one reading the poem will ever be affected. Then the last line, "Someone will have to go through that," lands with a strange mix of dryness and sympathy. The speaker doesn't say "we" or "our descendants." It's just "someone," a vague, faceless future person stuck with a problem we've handed on.
The speaker seems calm, even a little wry, but not cold. There's a quiet awareness of how small human concerns are next to cosmic time, and a hint of pity for whoever is around when it happens. The mood is deadpan, thoughtful, and gently unsettling, with the dash after "before" leaving a small pause before the bad news.