So tell me what you think it could be. His father? His parentage not what AS has been saying?
I think that perhaps one of the reasons AS cannot have him know the truth is that it would destroy her family and their very comfortable and "fulfilled" 😆 lives.
The one thing that CHANGED Dutta's life forever was SEEMA...her betrayal. It was the spark that led him to thirst after money and power...and his transformation from Dutta to Dutta Bhao. My supposition is that AS and Kala and Leela conspired to remove Seema from Dutta's life. She was already a vain, greedy girl...but they could not have their futures (the daughters' futures) negatively impacted by Dutta settling down. Remember AS saying to Dutta that he could not marry Seema, until he earns more money and helps with his sisters' situations?
Perhaps they did not know about the whole Don thing, or the violence that he would fall into.
Just a guess.
Another guess: his father was killed because of something related to AS or her daughters, or his father had killed himself, unable to provide for his family and their demands?
I'm sure we'll be surprised by the real secret, but it's fun nonethess to make up these different scenarios. Do write your guesses!