Take jellyfish galaxies, for example. They've got blobby bodies with long, tentacled arms of stars and gas trailing out behind them. Astronomers are pretty sure that jellyfish form when one cluster of galaxies slams into another. As stars collide with the gas that lies between galaxies within clusters, the force sends matter out in all directions. Super-hot, x-ray emitting gasses between the galaxies strip away stars and other gasses from inside, drawing them out into long, distinctive tentacles.