It is one of several (well, at least two) hypotheses proposed as putative explanations for the relevant observations. Hypotheses are more than placeholders for

It is one of several (well, at least two) hypotheses proposed as putative explanations for the relevant observations. Hypotheses are more than placeholders for theories; they are at least outlines of them, needing completion and verification. Proponents of dark matter have not defined it in terms of the observations it is supposed to explain; they have set out the known constraints on what it would be like, if it exists, but these constraints are not tight enough to amount to a definition. The language you quote disapprovingly is just a person acting like one, insinuating that the hypothesis they find more desirable or plausible is as good as settled, when it really isn't.