When two galaxies collide, what transpires is very different from, say, one billiard ball smacking into another. Instead of ricocheting away in opposite directions, galaxies are much more likely to meld together. After all, Ma points out, “Galaxies are mostly empty, so the stars and dark matter mostly just pass each other by. The chances of two stars hitting each other is tiny.” In fact, only one percent of the masses of these galaxies consists of matter we can see, such as stars and gases. The rest consists of dark matter-material we can’t see but astronomers have inferred from many observations must exist.

Actual galaxy mergers are hard to find and even harder to view. So Ma is doing the next best thing – simulating galaxy collisions using computer models. This way, she can specify the types of mergers she wants to analyze head-ons versus glancing blows; galaxies of different masses and shapes; even the occasional threesome – and analyze their fates with mathematical precision.