In sci.astro Uncle Al wrote:

By hypothesis and definition dark matter does not interact except by
gravitation.

Not true. By hypothesis, dark matter interacts weakly, but there is
certainly no requirement of no nongravitational interaction. In fact,
popular candidates (lightest supersymmetric particle, axions) certainly
do have nongravitational interactions. In fact, a major experimental
effort is going into searches for dark matter through such interactions.

Steve Carlip