The term "mysterious matter" is an expression that refers to something or someone whose characteristics, behavior, actions or origins are difficult to understand, explain or predict. Full definition
Survive a unique combination of platforming and bullet hell action in a vertical city overrun by mysterious Matter. (housemarque.com)
§ 3.5 Some 60 years ago, Dean William Prosser referred to choice of law as: «a dismal swamp, filled with quaking quagmires, and inhabited by learned and eccentric professors who theorize about mysterious matters in a strange and incomprehensible jargon. (slaw.ca)
In the game, the South Pole is covered by mysterious matter, and players are part of a crackerjack team sent to investigate - in that groovy outfit. (kotaku.com)