Asked by: Bazrry Cull, Taumarunui, New Zealand
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.
Read more:
- How do we know what the Milky Way looks like?
- How can the Universe expand faster than the speed of light?
SubscribetoBBC Focus magazinefor fascinating new Q&As every month and follow@sciencefocusQAon Twitter for your daily dose of fun facts.
