A MODIFIED theory of gravity could explain one of astronomy’s big mysteries: how spiral galaxies rotate without flying apart.
The favourite theory is that dark matter in the galaxies provides the required extra gravitational pull. But Israeli physicist Mordechai Milgrom had a different idea. In the early 1980s he came up with a theory called modified Newtonian dynamics or MOND, in which extra-strong gravity takes the place of dark matter.
The big problem with MOND has been that it contradicts the well-tested predictions of relativity, and so does not work in systems in which objects are moving at close to…
