What must I do to avoid sin after accidentally scratching another car and not knowing its owner?

To clear your conscience, you must search for the owner of the car you hit and compensate him for the damage, based on the assessment of experts or what you both agree upon, unless he absolves you. If you exert your utmost effort in searching but are unable to reach him, then strive to estimate the value of the damage in a way that you are most certain will clear your conscience, and then donate that amount on his behalf. There are similar cases in Islamic jurisprudence.