They’ve been both predicted and observed. Dark matter is a conjectural solution for an anomaly in the rotational speed of galaxies across their discs. In my opinion, it’s almost certainly a bodge, and if the W boson has a different mass to that expected it could account for what we call dark matter, as the implications go far beyond just the additional mass - if the mass is off, then perhaps the nature of the weak force is misunderstood - for instance, perhaps the mass variance is driven by a hidden variable, and that in turn could account for what we regard as “dark” phenomena.