Dark sector domain walls could explain the observed planes of satellites
  The observed `planes of satellites' around the Milky Way and other nearby
galaxies are notoriously difficult to explain under the $\Lambda$CDM paradigm.
Here, we propose an alternative solution: domain walls arising in theories with
symmetry-breaking scalar fields coupled to matter. Because of the matter
coupling, satellite galaxies experience fifth forces as they pass through
domain walls, leading to a subset of satellites with orbits confined to the
domain wall plane. We demonstrate this effect using simple simulations of a toy
model comprising point-like satellites and an infinite domain wall, and explore
the efficacy of various planarity metrics in detecting this effect. We believe
this is the first potential `new physics' explanation for the observed planes
of satellites which does not do away with dark matter.
