a priori is the wrong term because our physical theories are not a priori. Dark matter has lots of reasons to be assumed but a common one is that when we try t

a priori is the wrong term because our physical theories are not a priori. Dark matter has lots of reasons to be assumed but a common one is that when we try to explain the gravitational effects of galaxies based on a quantification of observable matter, there is a huge shortfall. That creates a space for something with a gravitational effect that is unobservable otherwise. That many of the objects of our physics are theory laden is true, but that they are therefore “unreal” or merely an artefact of theory does not necessarily follow.