There swarms of tiny robots all over the galaxy (this is the origin of dark matter), and when they detect abnormal radio signals coming from a star system they start to converge on that star system. We have been producing radio signals for about 100 years so they are well on their way to us.
When enough of them arrive, they descend on the origin of the radio signals and destroy all organic life.
Sheesh! I'm going to turn off my radio RIGHT NOW...(and that is a neat plot for a sci fi movie if it already has not been done).