The Universe
Explanation
The Joke
A character contemplates the vastness of the universe — billions of galaxies, each containing billions of stars, most with planets, all of it stretching across incomprehensible distances of space and time. They conclude that this makes human existence feel insignificant. Another character points out that "significance" is a human concept and the universe doesn't have an opinion, so we're free to consider ourselves as significant as we want.
The Humor
The comic engages with cosmic existentialism — the vertiginous feeling of insignificance triggered by contemplating astronomical scales — and offers a surprisingly upbeat resolution. The universe can't make us insignificant because "significance" requires a mind to assess it, and the only minds we know of are ours. It's a quiet rebuttal to the nihilistic version of cosmic awe.
Context
This is one of SMBC's more philosophical comics, engaging with the same territory as Carl Sagan's "pale blue dot" observation. But while Sagan used cosmic perspective to inspire humility, SMBC uses it to question the premise: if significance is a human concept, then cosmological scale is irrelevant to questions of meaning. It's an existentialist (rather than nihilist) response to the observable universe.