Alien species which is just about to ascend into being Pure Energy or whatever, but first they have to go through the galaxy and...
Alien species which is just about to ascend into being Pure Energy or whatever, but first they have to go through the galaxy and purge all their civilzational traces of memes and pop music and throwing up in the sink at 2 AM after getting shitfaced, so that the fledgling races millions of years later will only see how cool and exalted they were.