A mysterious smiling koala glimpsed by those who ascend or come close to death in Star Trek: Lower Decks, apparently bound up with the secret workings of the universe.
The Cosmic Koala is a running mystery in Lower Decks. It is first seen in Moist Vessel, when Lieutenant O'Connor finally ascends to a higher plane of existence. As he dissolves into energy he perceives that the universe rests on the back of a giant koala and wonders aloud why it is smiling.
The vision returns in First First Contact. Brad Boimler nearly drowns while helping to free the ship's cetacean officers and, on being revived, speaks of a koala he saw on the other side.
Later seasons return to the joke whenever characters brush against death or higher states of being. The series never explains what the koala is, and that is the point of it: it stands in for the grand cosmic answers that Star Trek's many ascended beings are said to find.