Earth That Was
Many stories and legends exist of the "Earth that was".
Many children grow up told it is a magical place, a place where humans live together in peace and harmony. A place that simply didn't exist once they came of age.
The most popular belief towards "Earth that was" is that it doesn't exist. It's a myth. A made up place to give humanity that is now spread out across the stars a commonality. A place we all ultimately came from. As there is no charted system for such a place, no archeological evidence suggesting it exists, people dismiss it as a fanciful tale.
There is a radical group, a violent/dangerous group, who belief "Earth" is a real place, and we as humans abandoned it. Humanity destroyed earths ability to hold life, and we were forced out onto the stars in an unnatural way. These people believe we're bad at our core, and that we just keep repeating the destruction and rape of planets over and over and over again.
Whether "Earth" is a real place or fictional one is up to each individuals belief.