What physicists call the “standard model of cosmology” holds, to put it crudely, that every galaxy is embedded within a halo of “dark matter.” The dark matter consists of particles that do not interact with electromagnetic radiation (hence the darkness) but that do have gravitic pull. This standard model has reigned since the late 1970s, when measurements of the velocity with which galaxies rotate found that the outside rim of a galaxy moves faster than it is ‘supposed to’ according to our usual understanding of physical laws.
Why is it going so fast? Because, the standard model says, the outer rim of a galaxy is being pulled around by that still-more-outer part, the dark halo.
Recent work by astrophysicists challenges the standard model. Some of this work strengthens an alternative view developed in 1983 by Mordehai Milgrom, that a modification to the law of gravity can account for the orbital rotation curve. Milgrom’s idea is known as MOdified Newtonian Dynamics (MOND).
Strange New Worlds:
In Milgrom’s view, Einstein’s view of gravity reduces to Newton’s in the matter of galactic rotation, so the “modification” is assigned to the 17th century theorist, not that of his 20th century counterpart.
One key point, though, is that if we accept this modification of Newtonian mechanics, we don’t need to accept the existence of those unobserved galaxy halos.
A book last year by philosopher-of-science David Merritt, A Philosophical Approach to MOND, looked at the contrast between MOND and the standard cosmology in some depth. The question remains open and contentious.