Ace Rent A Car pre-existing damage Richfield, Minnesota

I rented a car from ACE in Minneapolis on 7/11/2012 at around 10:30pm. When I get to the rental facility it's already dark outside. As with other rental company, a staff walked out with me with a small LED torch and checked the car for damage. Since it was a pretty new car and it was so dark outside, I did not notice any damage. I was very careful when driving the car and stayed mostly in the university district. When I returned the vehicle, they pointed out that there is a dent on the driver side and insisted I was the one responsible. I felt trapped as the car was rented to me late at night and it's normal to miss minor damage like this (even the staff and my girlfriend did not notice the dent at the time I got the vehicle). I insisted that I was not the one causing the damage and there was not way for me to notice such a small damage at night with only a small torch!!!!