The short answer is mystery. But the genuine answer is much more complicated than the one word.
Most people write off mysteries as being about murders. But the double-edged dagger of mysteries is not all mysteries are murders, and not all murders are mysteries.
Take shooting John Lennon. Many witnesses, and Mark David Chapman, admitted he shot Lennon, and plead guilty to the charges. Murder; but no mystery.
As for other mysteries, many have nothing to do with murder. Only recently was evidence found of the last resting place and what happened to Amelia Earhart. Buried treasures and sunken wrecks have fascinated people for many generations. But more importantly, to me, it is the human aspect of the mysteries.
In Secrets of the Gold, there are many mysteries as subsets of the one mystery. One of the main characters, Duff, has amnesia of anything prior to two years before the story starts. He has a wallet full of licenses saying he can drive any vehicle, but he doesn’t know why. His driver’s licenses list his home address, but he has never gone there for fear of what he might or might not find. In his jacket are secreted eighty-eight ingots of gold worth more than a million dollars, but he doesn’t know why. In his mind, he questions what kind of person walks around with a bulletproof jacket worth a million dollars?
Bean reaches a breaking point with living in foster care. It has bounced her from an abusive home to an abusive home. She sees a motorcycle packed for travel. In the café, she sizes up the biker. She needs what level of desperation to randomly put her life in the hands of a stranger? But the question goes both ways. How is Duff to know he can trust her not to kill him? It is a human drama people can relate to at an incredibly primal level. Most never see the question arise with such an acuteness… but dating? Getting married? Staying at a stranger’s house in a B&B? It is all there. Trust is a mystery we can all identify with.
The mysteries of the human nature and heart are the subtler, but no less powerful to write about. Sometimes, they can even beat out a good old-fashioned dead body.