Enigma (N.) – something hard to understand or explain

Synonyms: closed book, conundrum, head-scratcher, mystery, mystification, puzzle, puzzlement, riddle, secret, why, knock, misadventure, mischance, misfortune, mishap, tragedy

Source: https://www.merriam-webster.com/thesaurus/enigma 

Article: Linley, P. Alex, and Stephen Joseph. “Positive change following trauma and adversity: A review.” Journal of traumatic stress: official publication of the international society for traumatic stress studies 17.1 (2004): 11-21. 

Quote from article: “The enigma of health is just one small example from the range of problems which confront us.”

My thoughts/interpretation: We go through enigmas almost everyday. Some of which have been a question that need to be answered for thousands of years.