When two galaxies collide

If two galaxies were to collide, it's unlikely that anything in them would touch. Galaxies are teeming with billions of stars but they are also very, very large. This means these stars are spread out, very spread out. To give you an idea of how sparse they are, the average particle in the air is 500 times its own width from another particle. Our Sun is 29 million times its own width from the next nearest star. So when galaxies collide we certainly don’t see Hollywood style explosions; in fact, close up you probably wouldn’t notice much at all. The collisions are so tame that they are usually just referred to as ‘mergers’.