If the Universe is expanding, then why do galaxies collide?

The Universe started in the Big Bang and ever since then space has been expanding - increasing the distance between galaxies ever since. Galaxy mergers are also very common though - so how does this happen if galaxies are always moving apart from each other?

#bigquestions

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Dr Becky Smethurst is an astrophysicist researching galaxies and supermassive black holes at Christ Church at the University of Oxford.

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