Regardless of whether it was dark, your rear window was obstructed by dew or his bike was in the back of the parking spot, you backed into an object that you either didn't see or didn't notice. Any way you look at it your fault. It's not a heinous crime and you aren't a stupid person for doing it or anything like that. Everyone makes mistakes like this and luckily it didn't cause much damage and you will likely be more careful in situations like this in the future, so lesson learned there.
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