I am not able to gather any concrete information about the person who robbed me. How then do I forgive when I cannot examine this person’s life, including any trauma that might have contributed to this hurtful action?
We talk about taking three perspectives on the one whom you are forgiving: the personal perspective, the global perspective, and the cosmic perspective. The personal perspective is as you describe: trying to better understand the person’s own struggles, confusions, and wounds. Yet, you still can take the global perspective in which you reflect on the shared humanity between you and the person who robbed you. You both have worth, not because of your actions, but because each of you is unique and irreplaceable in this world. Depending on your spiritual/religious beliefs, you might consider the cosmic perspective: Are you both made in the image and likeness of God? Thinking in these ways may help you soften your heart toward the person.
For additional information, see Forgiveness Defined.