I have committed sins against people whom I don't know, and I find myself unable to ask for their forgiveness. I have sincerely repented and vowed never to harm anyone again. However, I have heard that when it comes to sins committed against third parties, their forgiveness is necessary. What should I do in this situation?
In the name of Allah, the almighty and all great.
Insha'Allah, In the permission of the almighty, I shall try my best to answer your question using fatawa.
According to Islamqa.info [Fatwa Link]
"If the person about whom you gossiped is dead or is not present, it is not possible to ask him for forgiveness, but the scholars said: you should pray a lot for him to be forgiven, and make du’a (supplicate) for him, and do lots of good deeds."
According to islamweb.net [Fatwa Link]
"If one could not find the person, one should continue asking forgiveness from Allah for himself and for the person who was backbitten. There is hope that Allah will accept the repentance."
May Allah support you and forgive you.
In the end, Allah knows best.