Mystery hit and run

I recently had some bad luck. I parked my car on the street and when I came back after work the side was all scraped up with something white and the side mirror was hanging by the cables. My insurance company thinks I drove my car into something concrete because the white stuff on the car doesn’t look paint, instead it’s powdery (like concrete). Also they say the damage extends almost 2 ft in height which is inconsistent with a drive-by side-swipe. I’m getting screwed over here because the damage is so unusual, but it wasn’t my fault. I know something hit my car but I don’t know how to prove it. Any thoughts on what could have happened to cause that kind of damage?

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