system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know the visible universe is 4% and not more or less?

I always hear how the visible universe as we know it is only 4% of everything else... but how do we know the visible universe is 4% and not more or less?  And how do we know the non-visible universe is 96% if we can't see it?  

Where do we come up with 100% percent of the everything in the universe and what all does it include?
gpt
The visible universe and the observable universe are two different things. The thing that is 4% is the collective mass of things in the observable universe comprised of ordinary (visible) matter. The other 96% is dark matter and dark energy, which are invisible but whose presence can be roughly detected via gravitational effects. From these gravitational effects, we have determined roughly how much dark matter and dark energy there are in the observable universe, and there's a lot. 

Specifically, dark matter impacts rotational velocities of galaxies, and dark matter impacts the rate of expansion of space. Measuring those allowed us to learn how much there is.