  I discuss open theoretical questions pertaining to the modified dynamics
(MOND)--a proposed alternative to dark matter, which posits a breakdown of
Newtonian dynamics in the limit of small accelerations. In particular, I point
the reasons for thinking that MOND is an effective theory--perhaps, despite
appearance, not even in conflict with GR. I then contrast the two
interpretations of MOND as modified gravity and as modified inertia. I describe
two mechanical models that are described by potential theories similar to
(non-relativistic) MOND: a potential-flow model, and a membrane model. These
might shed some light on a possible origin of MOND. The possible involvement of
vacuum effects is also speculated on.

Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology