It is worth noting that there is, as yet, no way to know whether the possible collision is definitely going to happen or not. The radial velocity of the Andromeda galaxy with respect to the Milky Way can be measured by examining the Doppler shift of spectral lines from stars in the galaxy, but the transverse velocity (or "proper motion") cannot be directly measured. Thus, while it is known that the Andromeda galaxy is getting closer to the Milky Way by about 120 km/sec, there is no way to tell whether it is going to collide, or miss. The best indirect estimates of the transverse velocity indicate that it is less than 100 km/sec. This suggests that the dark matter halos, although possibly not the actual disks, of the galaxies will collide. (less info)