We know that the universe is expanding but we do not know what force is causing that expansion, we label that specific unknown as black matter.
The difference is that in this case we have external evidence of that unknown (expansion of the universe). But what I wouldn't call knowledge is things that we have no indication of. In that case we have no reason to acknowledge the existence of an unknown that we have no indication about.
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I don't know much about science but as I understand , the expansion of the universe is caused by a force called dark energy , that is negative energy and not to be confused with dark matter, which is matter we can't see. And now you are saying that it is black matter ( not dark matter) which is causing the expansion?