[retube]

Visible mass is sufficient to explain the formation of stars and galaxies. What is a problem is the "rotation curve" of galaxies - that is orbital velocity around galactic centre as a function of distance from galactic centre. It's observed to be linear - ie stars far out are orbiting at the same velocity as stars closer in. In fact velocity should decrease with distance - see http://en.wikipedia.org/wiki/Galaxy_rotation_curve Dark matter has been postulated as an explanation - there must be other material that we can't see in the galaxy that results in the observed linear rotation curve.

Of course this assumes that the 1/r^2 holds for all distances. This is by no means certain, and it may well be that the linear curve is hint of new physics.

Edit: For other tantalising evidence that all is not well with 1/r^2, check out the problem with the trajectories of Pioneers 10 & 11:

http://moriond.in2p3.fr/J07/trans/wednesday/reynaud.pdf