In a new paper in the Astrophysical Journal, scientists led by Anshu Gupta try to understand this process using a combination of observations and computer models that add in the invisible effects of dark matter. By look at motions within massive galaxies that had already formed just a few billion years after the Big Bang, they were able to determine that the majority of stars had probably been cannibalized from systems that got too close and got eaten up. Since these systems had their own peculiar motions, the resulting galaxies are observed to have disorganized stellar motions. In what is one of my favorite quotes in a press release, Kim-Vy Tran, a co-author on this work, explains, “The surviving galaxies have grown fat and disorderly through incorporating smaller ones. I think of it as big galaxies having a constant case of the cosmic munchies.”

And really there is nothing I can say that is better than that. 

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Hungry galaxies grow fat on the flesh of their neighbors (ScienceinPublic)