  The Milky Way (MW) is surrounded by numerous satellite objects: dwarf
galaxies, globular clusters and streams of disrupted systems. Together, these
form a vast polar structure (VPOS), a thin plane spreading to Galactocentric
distances as large as 250 kpc. The orbital directions of satellite galaxies and
the preferred alignment of streams with the VPOS demonstrate that the objects
orbit within the structure. This strong phase-space correlation is at odds with
the expectations from simulations of structure formation based on the cold dark
matter cosmology (LCDM). The accretion of sub-halos along filaments has been
suggested as the origin of the anisotropic distribution. We have tested this
scenario using the results of high-resolution cosmological simulations and
found it unable to account for the large degree of correlation of the MW
satellite orbits. It is therefore advisable to search for alternative
explanations. The formation of tidal dwarf galaxies (TDGs) in the debris
expelled from interacting galaxies is a very natural formation scenario of the
VPOS. If a number of MW satellites truly are TDGs, mistakenly interpreting them
to trace the dark-matter sub-structure of the MW halo would significantly
enhance the 'small-scale' problems which are already known to plague the LCDM
model.
