[az09mugen]

My 2 cents : I have also a simple idea about time dilation explaining the dark matter, but I did not look out for numbers to check if it's plausible. Simply put : the center of a galaxy is the most massive part, often hosting a blackhole, and it's so massive that time runs slower than the outer part which is lighter, giving the impression on the outside (us looking from far away) that the galaxy rotates at the same speed in the center and in the outer part.