if space is expanding how do galaxies collide?

Get a group of a hundred or so people together in an open space. Tell them to run away from each other, but generally "outward". Many of them will bump into each other. Questions?

Gravity is a more significant force than the expansion of the universe when galaxies are close enough together.

The same reason that the solar system is not expanding. Things bound by gravity are held together by their interactions.

prosperity4all said:

if space is expanding how do galaxies collide?

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If a galaxy's motion towards another galaxy exceeds the rate of space expansion between them, the two galaxies will get closer. The closer they get, the less the effect of the expansion will be and the greater the gravitational attraction between them.