Scientology is what happened when a sci-fi author not even worth mentioning decided to turn a B-movie script into a religion. It is not widely spread on Earth or anywhere in the Galaxy as because of say, Zaphod Beeblebrox, Eccentrica Gallumbits and the Infinite Improbability Drive. This is because there are already enough weird things happening in the Galaxy without someone needing to make them up and believe them. This is downright crazy because we all know that the Infinite Improbability Drive is weird enough and we definitely don't need something weirder.
For humans, who are the main lifeform on Earth, this is excusable, because Earth isn't that weird to start off with. Or rather, not that weird when it comes to improbability. This is to say that not much very improbable happens there. But when it comes to life in general there, it really is weird. You know, video phones, digital watches and all that which they think are so technologically clever. Anyway, you've heard enough on improbability and so lets move onto Scientology. Life on Earth is so boring that they have to invent exciting stories to make up for this, which just shows. Here is one of those stories. It is also a scientological legend too and is in every way wrong.
Once, long ago there was a dictator called Xenu, far away in a distant galaxy. And his people were uprising against him. So as one does, he froze them in alcohol, put them in a spaceship and flew them to Earth where he stacked them around volcanoes and blew them up with hydrogen bombs. Some scientists say that this why humans are so stupid: they are mutated by the radiation from that time, four billion years ago. But as the story is wrong and is only believed by misguided maniacs like Tom Cruise, it is not to be believed.