Might our galaxy and Andromeda prospectively eventually be passing through a center of mass (CM) for almost contiguous respective dominant dark matter distributions for our small cluster (Local Group)? If there is any closing velocities of baryonic (luminous) matter, then would it be a tracer of such dominant DM contribution to a common gravitational field? Momentum of galaxies would pass through a center of mass, mostly of Dark Matter. Whereas for gravitational well, one would have more galactic deformation. But overall would the future of such 2 galaxies be repetition of such swinging back and forth, indefinitely? Contrarily, might there be no closing velocity for such 2 galaxies; yet still consistent with no center of DM motion?
R. Brent Tully
Astronomy: Collision course, 22 August 2012
Nature 488, 600-601 doi:10.1038/nature11482