What does Accelerator say to Dark Matter at the end of their fight?
「——ｙｊｒｐ悪ｑｗ」


Volume 15 (Old Testament), chapter Dark_Matter, section 4 (p. 320).

悪 (あく / aku) means "evil". The surrounding letters don't appear to gain any additional meaning in Japanese - they're probably just gibberish.