The first evidence that the Milky Way isn't a purely flat disk came from the Gaia Spacecraft. it has mapped the positions and motions of more than a billion stars, and from this we have some idea of our galaxies outer structure. For one thing, the Milky Way seems to extend outward more than we've thought, and the edge seems to have a rippled structure to it. Further analysis also shows a warp to the outer edge of the galactic disk.

The general consensus is that these are caused by ancient galactic collisions, such as the collision with the Sagittarius dwarf galaxy about 6 billion years ago. This new study argues instead that warped galaxies such as ours are caused by a tilted halo of dark matter.

We know that most galaxies, including our own, are surrounded by a massive halo of dark matter. Most of the mass in a galaxy is contained within this halo, so halos can affect the structure of galaxies over time. If the halo is tilted relative to the plane of a galaxy, the gravitational effects of the halo could skew the galactic disk. The question is whether this is significant or common enough to cause the structure we see in the Milky Way.

The authors begin by looking at data from the TNG50 run of the IllustrisTNG simulations. These are supercomputer simulations of galactic evolution that include cosmological evolution and dark matter as well as detailed magnetohydrodynamic interactions.