Are there two types of gravity?

摘要

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.