toaru kagaku no dark matter
A manga series of the Toaru franchise and the tenth manga series based off of Toaru Majutsu no Index. An original side story written by Kamachi Kazuma and illustrated by Kisaragi Nankyoku, it focuses on the past of Kakine Teitoku.
Its full title is Toaru Kagaku no Dark Matter (とある科学の未元物質, lit. "A Certain Scientific Dark Matter"), formally known as Toaru Majutsu no Index Gaiden: Toaru Kagaku no Dark Matter (とある魔術の禁書目録外伝 とある科学の未元物質, lit. "A Certain Magical Index Side Story: A Certain Scientific Dark Matter").
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This tag implicates toaru_majutsu_no_index (learn more).