Why do galaxies collide?

Galaxies do not live separately. The intensity of gravity holds them together. The galaxies communities are called clusters of galaxies.

In the central regions of these clusters are agglomerated a lot of galaxies that are moving quickly. They obey gravitational forces exerted by neighboring galaxies. The relative velocities between galaxies are about a few thousand kilometers per second.

The typical distances between galaxies in these areas are only about 5 million light-years. Collisions between galaxies are therefore relatively common. It is estimated there is a crash every few hundred million years in each cluster of galaxies.

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