system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] How is it that the Yuuzhan Vong exist outside the force?

I'm sure they probably try to explain this in the EU, but I'm not sure which book. I'm also not sure what the reason is behind the Vong not being able to be sensed by jedi.
gpt
They originated on a planet and lived in a symbiosis with their planet. When they were threatened and attacked by two species using machines etc. their planet sort of gave them biologically produced "living" weapons and technology with which they could defend themselves and ultimately defeat the enemies. 

Afterwards they started using their weapons, which their planet gave them, to start a conquest in their galaxy against other races to clean the galaxy of all machines. This lead to great destruction throughout the whole galaxy and eventually their connection to the force was severed.

To clarify this has nothing to do with the force only existing in our galaxy. But in extreme cases it is possible for individuals to have their connection to the force destroyed (also a powerful force technique). 

In this case it was caused through the destruction of their home-world during the war.

**EDIT:** 

Trying to make it more clear.

The reason Jedi cannot sense them is that they leave no signature or trace or presence in the force. As it is described the Force flows through you is inside and around everyone and connects all living beings (accept those who lost their connection). Because of this nobody can sense someone if they have no connection as they leave no "footprint" in the Force.