But it supposedly couples to gravitation, and should then coalesce into dark matter galaxies, stars and planets, which would definitely seem to have an effect, crashing into each other, and presumably ordinary matter objects too. Yet, we don’t see any evidence of that.
I’m not a big fan of plugging something in just to make it fit the standard theory.
I’m not a fan of the DM theory, either. I think it’s just a variable they stick into relevant equations to make them come out right. It’s not a physical entity, and they’ll never ‘discover’ any of it. In the meantime, we’ll just go with what works.