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I am no expert, but I was under the impression that astronomers were fairly confident that dark matter could not possibly all be ordinary, but invisible things (like un-illuminated 'Jupiters' and anitmatter and such). I do not understand the details of exactly why, but I thought that they were confident that there is simply not enough of the ordinary stuff to explain gravitational observations - hence the mystery of its nature. But I could be wrong about that.