As you can see in the "Noir" section of this blog, I've been happily commenting on various films as examples of film noir. In doing so, I've relied on Otto Penzler's definition.
Now I learn there's more to it than that---much more. In the table of contents for the Wikipedia article on film noir, the first item is "Problems of definition." It turns out there are so many problems, and they are so complicated, that, "There is no consensus on the matter."
Yet the article tells us that in film noir the worldview is pessimistic, the morality is ambiguous and the tone is grim. Penzler says essentially the same, and this article refers to Roger Ebert's description, "A movie which at no time misleads you into thinking there is going to be a happy ending."
All this sounds depressing, yet film noir (and neo-noir such as Chinatown, Raging Bull, Basic Instinct, Body Heat, Pulp Fiction, Fight Club, Training Day and many more) remain popular for the same reason tragedy has always been popular (Oedipus the King, Hamlet, A Doll's House, Death of a Salesman).
We hope that Good conquers Evil and that Love is the answer, but we know those statements are are not always true. Too often in life a force greater than ourselves overwhelms our best efforts. The Greeks called it fate. The twentieth century called it socio-economic conditions.
We watch a play or a movie about an imperfect man or woman who struggles for happiness, knowing all the while they will lose. We suffer with them as they fight the good fight. When they lose, we experience a purgation of fear and pity, which Aristotle called catharsis. And we feel better.