Supergirl: Universe End by Mariko Tamaki

After joining the Superman Family and their heroic efforts in Metropolis, Kara thought she had found her place in the world. That is, until Power Girl's sudden reappearance made her redundant, putting both her identity and role into question. To find the answers she seeks, Supergirl must pursue a mysterious threat that only she can handle by going on a soul-searching adventure to the farthest reaches of space at the universe's end!