Most of the comments thus far assume that both galaxies are composed of normal matter and have no prior "history" together. NASA has no way of knowing this, nor do they know other relevant details - like the 'type' of matter.
One simply cannot discuss this subject intelligently without reference to the disputed work of R. Foot, who is kind of the R. Crumb of cosmology. http://arxiv.org/find/astro-ph/1/au:+Foot_R/0/1/0/all/0/1 When we have discussed this topic previously, Horace believes that mirror matter - MM - has negative gravitational charge, as does Foot - so this outcome of 'recombination' would be prohibited and in fact NASA's evidence for gravitational linkage is wrong. But at this point it is all conjecture, even at the highest levels - so to speak. IOW - NASA's data for all of this is weak. Some other possibilities: 1) The milky way and Andromeda are incredibly similar in size, shape, mass, apparent age, number of stars, spectroscopy, and many other physical properties... which point to something being "mirrored"... but could also be coincidental. It could point to a common origin ("little bang") to which they are both returning, over time, due to gravitational linkage at a common nexus. 2) Another implication is that paired bodies begin as matter/mirror-matter pairs, and that the return results in something akin to annihilation of both, or at least a common starting point. This assumes only one kind of gravity. Matter/antimatter pairing may not be ruled out as the alternative explanation to mirror matter, but there are reasons to suspect that Andromeda is not an antimatter galaxy. 3) Mirror matter is pure conjecture but it is probably preferable to antimatter - since it suggests a different outcome than annihilation. 4) If the visible mass of Andromeda and the MW are supplemented by 90% more mass in the form of dark matter, then any relative "peaceful coexistence" in the far future is out of the question. 4) No one has a clue, really. Jones
