It seems that Humanity has spread all over the universe and colonized many planets. I guess that, originally, all humans come from the same place, Earth or whatever. However, I don’t know how to justify the big differences in technology levels between planets, or how the low technology cultures arrived to a given system using slipstreams.
How do you explain it in your games?
This is how I’d handle it:
The great expansion took place before anyone from Earth had developed the slipstream drive. Humanity crawled to the stars in ships that could only approach the speed of light but never exceed it.
Some traveled on generation ships but most used cryogenics to suspend their life functions for the trip.
When they arrived at the planet they were aiming for, they had few resources beyond those they found on their new home and had to build new civilisations almost from scratch.
Some planets were rich in materials, some little more than deserts.
Some were devoid of life forms above level 3, others had native life or other colonists competing for the same resources as the colonists.
Each colony had to struggle back to the stars at its own pace. A few had enough functioning technology with them that it took them months, but most took decades, centuries or never made it on their own.
As time passed, many colonies forgot where they came from and experienced severe culture shock when first contact was made with other humans.