If Galaxies Are All Moving Apart, How Can They Collide?

This Hubble Space Telescope image shows a collision between two spiral galaxies, NGC 6050 and IC 1179, in the Hercules constellation. NASA, ESA and HUBBLE HERITAGE

From Scientific American:

Cosmologist Tamara Davis, a research fellow at the University of Queensland in Australia and an associate of the Dark Cosmology Center in Denmark, brings together an answer:

The dynamics of the universe are governed by competing forces whose influence varies with scale, so local forces can override universal forces in discrete regions. On scales larger than galaxy clusters, all galaxies are indeed moving apart at an ever increasing rate. The mutual gravitational attraction between two galaxies at that distance is too small to have a significant effect, so the galaxies more or less follow the general flow of the expansion. But it is a different story in a galaxy's local neighborhood. There the gravitational attraction can be very significant and the interactions much more exciting.

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