So just before this topic dies naturally, or is closed, I am going to include here some of my remarks about why it is not surprising that in light of the astrophysics evidence we have, scientists prefer to posit the existence of dark matter rather than modify gravitational laws like @Joe_Bakhos or even MOND does. This is adapted from what I wrote to Joe in a PM in response to his more specific remarks - I thought it would be beneficial to have it accessible to everyone else.
This shouldn’t be taken as an authoritative opinion universal to all physicists, but merely one physicist’s take on why the idea doesn’t seem far-fetched. In the course of writing this post I came across this excellent recent review article (Bertone & Hooper, History of Dark Matter) which has a more complete picture of how this paradigm came to be developed. I learned several interesting facts from this paper. If Joe or anyone else interested in learning in greater depth how dark matter became the dominant paradigm, please read this paper instead.
You can read my summary here: Why do Scientists Believe in Dark Matter?.
[@moderators: removed duplicate content, find it at the inserted link]