50 Cent -- NOT THE FATHER Of Baby #2 ... At Least On Birth Certificate

50 Cent didn't father a child with ex-GF Daphne Joy -- the woman he allegedly attacked during a domestic violence incident last month -- at least according to the child's birth certificate ... but the document doesn't rule out 50 either. Not even close.

TMZ obtained the certificate -- which shows Daphne is the mom -- but it doesn't list a father. The daddy field is blank, which means one of two things ... A) Daphne didn't know who the dad was, or B) she didn't want to say.

But all clues still point to 50 being the dad -- namely, the child's last name is listed as Jackson ... as in Curtis Jackson, aka 50 Cent.

As we reported, neither 50 nor Daphne have ever acknowledged the child is his ... but the L.A. City Attorney's Office kinda let the cat out of the bag when it announced the domestic violence charges against 50, saying he and Daphne had a "child in common."