I think what they mean is that “dark matter” is just a name that scientists gave to a phenomenon they have yet to understand. It’s a variable in a math problem that represents something that’s there, that influences the whole system, but we don’t quite yet know what it is.

If my memory is correct, one of these problems is the mass of the universe. It doesn’t quite all add up. So they made up this dark matter to explain it. It’s the missing matter that we can’t quite observe but makes it all make sense.