This article states, “Several billion years from now, scientists predict, the galaxy and the Milky Way will collide. . . .” How can galactic collisions occur in an expanding universe, where galaxies should be moving away from each other?

James Hendry
Florissant, Mo.

On the large scale, objects are indeed receding from each other. But locally, the gravity of two large galaxies that are relatively near overcomes cosmic expansion. —R. Cowen

From the Nature Index

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