nothing
Explanation
The Joke
Two abstract, donut-shaped beings float in a void discussing the nature of nothingness. One asks why there is nothing and not something. The other asks what "nothing" means to the first one, and it replies it would like there to be "stuff." The first being argues that "stuff is impossible," and when asked why, explains: "Because if there were stuff, there wouldn't be nothing." The other says "that seems a bit circular," to which the first replies: "It can't be circular because circles don't exist." The other concedes: "Touche."
The Humor
The comic inverts the classic philosophical question "Why is there something rather than nothing?" by placing it in a universe where nothing exists (except, paradoxically, the beings having the conversation). The humor lies in the circular logic — the being argues stuff is impossible because its existence would contradict the current state of nothingness, which is a tautology, not an explanation. When called out on the circular reasoning, the being cleverly deflects by noting that circles themselves do not exist in a universe of nothing — so the reasoning cannot be "circular." This is a joke about philosophical reasoning that eats its own tail: every objection is absorbed into the framework of nothingness. The visual gag of donut-shaped beings (which are themselves circular) discussing whether circles exist adds another layer of irony.
References
The comic plays on the fundamental metaphysical question "Why is there something rather than nothing?" attributed to Gottfried Wilhelm Leibniz and explored extensively in philosophy and cosmology.