It's midnight here so I'm gonna sign off and rest a bit, but as a final thought here's one more compelling piece of evidence in favor of dark matter and against alternatives.
It has been proposed that dark matter is not actually there, but rather our picture of gravity is simply incomplete. MOdified Newtonian Dynamics (MOND) says that for masses with very low accelerations, the laws of gravity look different than Newton's law that we always use.
It was developed to explain the flat rotation curves observed by Dr. Vera Rubin, here's that thread from earlier if you missed it:
Stars in the far reaches of a galaxy fall into this regime, so MOND basically says dark matter isn't there, our equations are just wrong. One observation in particular makes that very hard to believe, and reaffirms that dark matter really does explain a great deal.
When galaxies collide, there is so much space between stars that the stars don't actually interact. Remember too, dark matter just passes right through things. So this leaves only gas clouds to actually collide and interact since they're bigger and more diffuse than stars.
If dark matter weren't there, when observing this collision the "missing" mass on account of incorrect gravity equations should lie with the bulk of the light-producing mass, which is the interacting gas. If DM is not there then there's no actual other mass to worry about, right?
Wrong. Behold! A NASA image of the Bullet Cluster, two collided galaxy clusters. Whole galaxies on either side have passed on through, while the gas (colored pink) in the middle is lagging from having interacted. That gas is the bulk of the normal mass. What's the blue region? 
The blue region is where lensing measurements imply most of the mass actually is. Which wouldn't be the case unless there were really something there and we can't see it! Lensing thread for reference:
So that's why even in the face of other theories that sometimes make points, dark matter is still far and away considered to be our best bet.
