Oh - funny question with more disclosure. My first thought was "dead" but in reality it should be "not known to me".
Another opportunity for disclosure - I never knew my father. My mother had an affair with her (married) landlord and got pregnant (with me). His wife was pregnant at the same time and also had a girl (how sweet). I know his name - I asked my mother once when I was doing genetics in med school. He (if he is still alive) and presumably his family live in town. I have from time to time thought about looking into it, but have decided it's not worth it.
All three of my mother's children have different fathers. The last was my abuser.