When galaxies collide, and several have merged into our own Milky Way, there are imprints that show as streams of stars, which are all moving in the same direction, but not yet equilibrated into the usual spiral arm rotation.
One such stream, the S1 stream, is of particular interest since it crosses the path of our sun. Thinking about the dark matter from the merged galaxy, this would imply that there is a dark matter “wind” blowing past us at about 500km/s. This could potentially help in our efforts to detect the components of dark matter in earth-bound experiments.
However it is possible that the dark matter from a merger would follow a different trajectory from the stars, since we know that the dark matter in a galaxy does not occupy the same geometric shape as the stars in that galaxy. Indeed dark matter may not have the same equivalence of gravitational and inertial mass.
The overall grand structure of the universe is that galaxies are scattered along filaments surrounding voids as illustrated in the simulation below:
Two studies, one from 2013 and one from 2017, now confirm that not only is our own galaxy, the Milky Way, located atypically in a void rather than a filament, but also that it is located in the largest known void – about 7 times larger than an average void.
The original study came to this conclusion on the basis of galaxy catalogues, whereas the new study measures the motions of galaxies by measuring changes to the energy of Cosmic Background Radiation which has passed through those galaxies. The motions of galaxy clusters indicates those regions of higher gravitational attraction. The two studies, by different means, are consistent.
This, statistically, is a very surprising result. It might suggest that there is something wrong with the underlying theory behind astrophysical measurements, it might be a statistical fluke, or there might be an, as yet, unthought-of explanation.