Full Idea
Whatever dark matter is made of, it must have mass to feel and generate gravity; but no electric charge, so it does not interact with light. The leading candidate has been the weakly interacting massive particle (WIMP), much heavier than a proton.
Gist of Idea
Dark matter must have mass, to produce gravity, and no electric charge, to not reflect light
Source
New Scientist writers (Why the Universe Exists [2017], 08)
Book Reference
New Scientist writers: 'Why the Universe Exists' [John Murray 2017], p.150
A Reaction
Note that it must 'generate' gravity. The idea of a law of gravity which is externally imposed on matter is long dead. Heavy WIMPs have not yet been detected.