What must I do to absolve myself after causing damage to a car and not stopping, knowing that I cannot find the owner of the car now?

One must repent to Allah and frequently seek His forgiveness, and search for the car owner to pay him his due, because redressing grievances is a condition for repentance. If the owner cannot be found, then one should frequently seek forgiveness and perform good deeds, for dues will be settled from good deeds on the Day of Resurrection.