If the Universe is expanding, why is the Andromeda galaxy on course to collide with the Milky Way?

The expansion of the Universe is a large-scale phenomenon: in general, the further away a galaxy is, the faster it recedes from us. But over small regions of space, this expansion is negligible compared to the motion of individual galaxies. The Andromeda and Milky Way galaxies are sufficiently large – and sufficiently close together – to create a gravitational force that overcomes the general expansion and pulls them together. But don’t worry, the collision won’t happen for another four billion years.

Four billion years from now, the night sky could look like this