Andromeda to collide with us

Dennis Overbye writes in the NY Times:

The Milky Way and Andromeda are the dominant members of a small family of galaxies known as the Local Group. Whereas the universe is expanding and galaxies are generally getting farther and farther away from one another with time, the galaxies in the Local Group are bound together by family ties in the form of their mutual gravity. Our relatives aren’t going anywhere.

And there is the problem. Andromeda and the Milky Way are actually heading toward each other in the do-si-do that constitutes life in a galaxy cluster. Recent measurements with the Hubble Space Telescope have confirmed that they will hit head on in about two billion years. Since galaxies, like atoms, are mostly empty space, they will pass through each other like ghosts, but gravity will disrupt the stars and strew them across space in gigantic spectacular streamers. Eventually they will merge into a single giant galaxy.

Does this make any sense to you? Atoms are not mostly empty space. Solids certainly do not pass thru other solids like ghosts.

If Andromeda passes thru us like a ghost, then it would not merge with us. It would continue for at least another 2 billion years, and maybe 20B years. If Andromeda is really gravitationally bound to the Milky Way, then it would eventually slow down, reverse, and collide with us again. But then it would just pass thru like a ghost again. That could continue millions of times, unless dark energy drives the galaxies permanently apart.

I am guessing that the galaxies will not really pass as ghosts, and maybe millions of stars get knocked out of position. That could dissipate some energy and make is possible for the galaxies to merge, but surely the merger would take many billions of years, and most of the stars would probably burn out first.

Overbye probably called some expert who has all this figured out. We should have gotten a better explanation.