The planet was completely isolated from all civilization for well over one thousand years until a stray cluster portal created a large gravity wave that carried this planet's star to the milky way, where it is to this day.
When wormhole travel first came to be, it was not uncommon for entire ships and sometimes even fleets to seemingly disappear.
Little did humanity know, that the ships were transported up to a little over a trillion lightyears away, deep within galactic voids. Over one thousand ships were lost this way.
Over 13 Billion years ago, a young star in an unknown galaxy was thrust out of the galaxy at almost twice the speed of light. It is a mystery how it could go this fast, one theory is that an ancient civilization did this.
The star has been coasting on a gravity wave that has slowly been losing energy for billions of years, so that theory makes the most sense.
The planet was discovered by one of the ships that was lost.
Fortunately for the inhabitants, the ship was built for colonization. The planet was colonized, but it slowly devolved into tribes spread across the original continent.