Question: What happens when galaxies collide with one another?

Anna Scaife answered on 13 Nov 2014:

Hey Louise – great question. Lots of things happen!

Firstly galaxies are full of gas and dark matter. Dark matter is strange stuff and the only force it reacts to is gravity, but galaxies have a lot of gravity and so when they crash into each other all the dark matter gets sloshed around.

The dark matter itself has mass so it exerts a gravitational force on the gas, which then tries to follow the sloshing dark matter. At the same time there are other forces between the gas in the two galaxies, which can pull the gas in different directions, and often one galaxy can strip the gas out of the other…

Another thing that happens is that, because the galaxies are moving they have lots of kinetic energy and when they collide that energy has to go somewhere and often it goes into the gas molecules which then have chemical reactions and trigger new bursts of star formation!