It fits within the subgenres of post-apocalyptic fiction and space opera.
Here's why:
* Post-apocalyptic: The story takes place after a devastating event that has drastically altered the universe.
* Space Opera: The story involves space travel, advanced technology, galactic empires, and epic conflicts on a grand scale.
Therefore, while it's primarily classified as science fiction, "Last in Universe" also draws elements from post-apocalyptic and space opera genres.