have we ever seen galaxies collide?
there are several examples of colliding galaxies
here's one of many examples ....
The Antennae Galaxies (also known as NGC 4038 and 4039) are a pair of distorted colliding spiral galaxies about 70 million light-years away, in the constellation of Corvus
Google is your friend:
I'd be interested to know what the dark matter around the galaxies do when do of them come together.
Google "bullet cluster"
does the collision emit lots of energy like a supernova
There is a lot of heat generated by merging dust clouds. Suns and planets barely notice each other. I'd guess nothing anywere close to a supernova.
Although galaxies look like solid objects they are largely empty space and the distances between star systems and other massive objects is huge.
The chances of two very massive stars colliding is really low, but not impossible, if it happened a one-off supernova might result.
The chances of multiple stellar collisions happening continually whilst the galaxies merge is so low that it might as well be zero.
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