About twenty millennia ago, there was a cataclysm of galactic proportions. Geological evidence on all known worlds confirms that it was a catastrophe that affected all of space pretty much simultaneously. Worlds were hit by killer earthquakes, monstrous tidal waves, and other natural disasters. They were the lucky ones; some planets were shifted in or ripped from their orbits, some were torn to pieces, and some solar systems were entirely obliterated. Most life forms that existed at the time were wiped out. There is no evidence that indicates what caused the cataclysm and if or when it will happen again.
As you might imagine, there is a great deal of speculation on what exactly the cataclysm was. The prevalent theory among cosmologists is that it was some energy ripple left over from the collision of the two original galaxies that make up Corona Galaxy, but this idea is sheer speculation. There is no evidence of this or any other theory being true. There are also rumors of the odd pre-cataclysmic artifact, but if such things exist, they may be hidden or just lost. Once again, it is a great mystery.
