  An ultra-light axion with CP violating interactions with a dark sector and CP
preserving interactions with the visible sector can act as a novel portal
between dark matter and the Standard Model. In such theories, dark matter
sources an axion field extending over the entire galaxy, the gradient of which
can be searched for with precise spin precession experiments. A
reinterpretation of existing co-magnetometer data already constrains theories
that are consistent with astrophysical bounds, and near-future experiments will
begin probing well-motivated models. The required interactions can arise from a
confining hidden sector without necessitating fine-tuning of the axion's mass.
