The dark sector may not be completely dark, since a dark particle can still couple to the photon via a milli-charge, or higher-dimensional operators. The talk will focus on a light dark particle around and below the GeV-mass scale, and discuss its potential electromagnetic properties, and experimental signatures in intensity-frontier, colliders, and astrophysics. In case that this light particle is dark matter, it will become subject to constraints from direct/indirect detection and cosmological probes. Theoretical issues, regarding the validity of the effective operator approach, will be discussed too.