Imagine you're on a rock in a solar system for thousands of years, you develop a civilization and just as you're starting to explore a night sky you realize another galaxy will collide with the one you're in. Years and years of preparation and you've calculated your planet will be ok, but your entire solar system will change trajectory away from the galactic center. Any hope you had of finding life in a nearby solar system is gone. Your civilization will never be trapped, alone, on this one rock for eternity.