Full Idea
Gravity is an odd sort of force, not least because it only ever works one way. Electromagnetism attracts and repels, but with gravity there are only positive masses always attract.
Gist of Idea
Gravity is unusual, in that it always attracts and never repels
Source
New Scientist writers (Why the Universe Exists [2017], 05)
Book Reference
New Scientist writers: 'Why the Universe Exists' [John Murray 2017], p.99
A Reaction
This leads to speculation about anti-gravity, but there is no current evidence for it.