Of course some cosmologists say the whole dark matter thing is rubbish Proponents of MoND theory suggest that by modifying Newton's theory of gravity you can do away with the need for the dark matter entirely. http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
There is a great article in the New Scientist of may this year called "It is time to give up on dark matter".
There seems to be an alternative called Modified Newtonian Dynamics (MOND).
A single MOND formula is said to perfectly account for every discrepancy current theories invoke dark matter for.