Large Triangle

This event has been bugging me for years. 
I was driving back to my parents house with my girlfriend at the time. We had just been out on a date, and it was a warm evening outsude. I was driving my mother's minivan, and we had the windows down because it was nice out.  The road my parent's house in on was, at the time, in a semi-rural suburban area.  It was a very clear night with lots of stars, no moon from what i recall.
 When we were about 2 miles away, I saw a dark shape above the treeline in the direction we were headed. Even though it was very dark, there was still some ambient light from philadelphia on the horizon below the treeline. Because of that, I could define space that was much darker then the surrounding sky. I pointed the shape out to my girlfriend, and we both thought it had to be a cloud. Then we realized that it seemed to be moving. Then we thought it must be some sort of flat looking blimp. We could see no lights on it whatsoever, which we thought was very odd for an aircraft at night.
 When we were approximately 1/2 mile from home, we saw that the object was much larger and closer then we thought, and then it was moving over us. I slowed the car down to almost a stop and stuck my head out the window to see what it was. It made no noise at all, and blocked out a very large portion of the stars. i was able to see that it was roughly triangular in shape- though i can't be certain of that fact because it all happened so fast and unexpectedly. I didn't see any visible markings or lights, and it wasn't moving at a high rate of speed.
We both sort of laughed the whole thing off and never talked about it again.
 I have been at a loss ever since to explain what it was, since aircraft flying at night have some sort of lights. I'd also expect some sort of noise, even from a low-flying blimp- should one decide to float around in the dark at low altitudes.
Witness indicates that the date of the incident is approximate.  PD