I would try to take a sample of the dark matter and see if there is a
solvent that might work. There is a product call "bug and tar remover"
that seemed to work pretty well when tar from a new road got on the
lower half of my rear bumper.
Hope this helps you.
this may be a long shot, but if the car was parked outside, some of the
bird droppings can be very liquid and resembles a dark fluid that is
difficult to remove after - it leaves a stain that comes out ohh so slowly
with rubbing compound
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