The theory of gravitation is not at question here, it is in fact the basis for proposing the existing of some yet-unseen matter to account for why the observed

The theory of gravitation is not at question here, it is in fact the basis for proposing the existing of some yet-unseen matter to account for why the observed galaxies behave. Someone proposing that there is no such thing as the thing we have never observed is... a pretty "baked" argument.