Missing mass in galaxies and clusters of galaxies

A look at the other papers in this volume will show the present one to be singular. Dark matter is a prevalent paradigm. So why do we need to discuss alternatives? While observations seem to suggest that disk galaxies are embedded in giant haloes of dark matter, this is just an inference from accepted Newtonian gravitational theory. Thus if we are missing understanding about gravity on galactic scales, this inference may be deeply flawed. And then we must remember that, aside from some reports which always seem to contradict established bounds, DM is not seen directly. Finally, were we to put all our hope on the DM paradigm, we would be ignoring a great lesson from the history of science: accepted understanding of a phenomenon has usually come through confrontation of rather contrasting paradigms.

To construct a competing paradigm to DM, it is best to bear in mind concrete empirical facts. Newtonian gravity with the visible matter as source of the Poisson equation properly describes all observed systems from asteroid scale up to the scale of globular clusters of stars (∼105 stars bound together in a ball the size of a few tens of light years). But as we move up to galaxies, ours or external ones, troubles appear. In essence, the way that disk-like galaxies rotate is incompatible with the Newtonian gravitational force generated by only the visible stars, gas and dust.