Not knowing my Dad has made me feel different at many points in my life

When I was younger it really confused me. I would be asked by friends 'Where's your dad?' and in my mind, it didn't make sense that I didn't have one, so I never really knew what to say. I have never seen a picture of him, never read a letter from him, or met him. He is an idea to me, not a reality and it makes me feel different compared to my friends' who have known their Dads.