I'll admit my knowladge on the subject  is intensly limited but isnt the theory of dark matter a little bit of a long shot?  While newtons laws have served faithfully for years and years and over and over whose to say stars and galaxies simply jsut dont act different because of the chemical components or the way gravity acts on them all in space? Scientisits observations could very well be wrong also.

 If the sun are made up of different chemicals they would have different masses for the same size.  Wouldnt this affect the way gravity is acting on them and thus the way they orbit each other?  Once again I'm very aware physics is not my strongest science so someone correct me if I'm wrong.