x1 is a quantity of dark matter/dark gravitation [unknown thing with gravitational consequences] with properties x2
If scientists eventually figure out that dark matter is not a thing at all but, for example, the result of a mathematical correction to our present understanding of gravitation, this word may need to be redefined to something similar to "x1 relates to dark matter/dark gravitation in manner/aspect x2", or whatsoever is fitting, or it could be deleted all together.