  It was found that satellites of nearby galaxies can form flattened
co-rotating structures called disks of satellites or planes of satellites.
Their existence is not expected by the current galaxy formation simulations in
the standard dark-matter-based cosmology. On the contrary, modified gravity
offers a promising alternative: the objects in the disks of satellites are
tidal dwarf galaxies, that is small galaxies that form from tidal tails of
interacting galaxies. After introducing the topic, we review here our work on
simulating the formation of the disks of satellites of the Milky Way and
Andromeda galaxies. The initial conditions of the simulation were tuned to
reproduce the observed positions, velocities and disk orientations of the
galaxies. The simulation showed that the galaxies had a close flyby 6.8Gyr ago.
One of the tidal tails produced by the Milky Way was captured by Andromeda. It
formed a cloud of particles resembling the disk of satellites at Andromeda by
its size, orientation, rotation and mass. A hint of a disk of satellites was
formed at the Milky Way too. In addition, the encounter induced a warp in the
disk of the simulated Milky Way that resembles the real warp by its magnitude
and orientation. We present here, for the first time, the proper motions of the
members of the disk of satellites of Andromeda predicted by our simulation.
Finally, we point out some of the remaining open questions which this
hypothesis for the formation of disks of satellites brings up.
