I opened my car door and i think i bump/dented someone's car?

It sounds like you checked the corresponding location on their car where your door would have hit, and there was no damage, thus, you are o.k.

In general, there is no basis for legal liability if there is no harm or no damage. Your question suggests that you are not sure if there was any damage or harm because of the weather conditions and because of other preexisting damages on the vehicle that was accidentally hit by your door. In fact, you state that you feel really bad about what happened because you really aren't sure if there was any harm or damage. I will presume that after you attempted to inspect the other car, you simply left the scene. That is why you are now feeling badly, because you think you should have done something more. Going to the Police, at this point, would not be appropriate. It is too far after the event to make this a reasonable choice and you don't have the particular information needed (make/model/year/license number of car; name/address/phone/operators license number for the owner) for the Police to investigate this potential matter. First, you should have obtained a witness to, or a photograph of, the location on the other vehicle where your door made contact. Such evidence would prove whether or not your door caused any damage to the other vehicle. this could have been done by you (is there a camera in your cell phone? or someone in the parking lot whose name, address and phone number you obtain?) or by the management/security personnel of the business whose parking lot was the scene of this accident. Let me suggest that you should next have done one of two things before leaving the scene of this accident. If you could have waited for the owner of the other vehicle to return to the scene, then you should have done so to determine, with the owner of the other car present, whether or not your door caused any damage or harm to the other vehicle that was not already on the other vehicle. If so, then all necessary contact and insurance information could have been exchanged between you. If you could not wait for the owner of the other vehicle to return to the scene, then you should have advised the management/security personnel of the business whose parking lot was the scene of this accident what had happened and who you are. This way, you would not have left the scene of an accident without making a report of same. Alternatively, you could have left a note with your phone number on the vehicle, but in this day and age I would not recommend this as being a very prudent option. In the end, your mistake was two-fold: (1) leaving the scene of an accident without confirming whether or not there was any damage caused by the accident; and, (2) leaving the scene of an accident without making a report of same.