The phrase "mysterious part" is an idiomatic expression that refers to something or someone who has a mysterious, hidden or unknown quality. Full definition
It seems that the most mysterious part of the hiring process is salary. (careerdigital.com)
«The human brain and thinking are the most mysterious part to me,» she says. (sciencenews.org)
It's a startling thought that dark matter, now such an integral if mysterious part of the modern picture of the universe, might simply vanish overnight. (discovermagazine.com)