How can I keep my illustration completely full? i see that after i fill the color very thin lines appear around the fill. how do i fix this?
There might be a stroke applied to the objects.
It would be helpful to see a screen capture with the object selected, including the stroke and fill icons in the tools panel.
Could you add a screendump? If you have two colours next to eachother say red meeting blue you may get a thin appearance of a boreder due to anti aliasing. This is the computer smoothing the pixels on either side but the smothing leaves an apparent hairline. Antialiasing was to fake higher resolution when monitors had few pixels, on a retina monitor it is not as necessary. Turning off anti aliasing may make these kind of artifacts dissapear.
If it is this problem or another is hard to tell without a screen shot.
Yes that's anti aliasing. (Mathematica rounding off Numbers)
how can i fix this? turning off anti aliasing?
Under your general preferences
You can try this and see whether it helps: Hold Ctrl/Cmd and press E.
This toogles between GPU and CPU, the former sometimes moving in mysterious ways. You can always toggle back.