Approximately 100% if you assume the cosmic microwave background to be actually a "background" and not an object on its own rights.

This is why the sky is black in the first place.

With the very few exceptions of nearby objects (Earth, Moon, Sun, ISS and some nebulae) anything else is point-like.

If you insist on making longer expositions, you will "spill" some initially invisible objects into whole pixels, but this is not fair - they are geometrically much smaller than the pixel in question.

Galaxies (including, but not limited to, the Milky way) represent themselves as an object with a shape - again - only because of our limited resolution. Otherwise, they are quite sparse and transparent. Stars themselves are quite bright, but the whole galaxy - not much.

Approximately 100% if you assume the cosmic microwave background to be actually a background and not an object on its own rights.

This is why the sky is black in the first place. It consists of stars and a background.

With the very few exceptions of nearby objects (Moon, Sun, ISS and some nebulae that don't add up for a whole tenth of a percent and if you don't consider Earth as "sky") anything else is point-like.

If you insist on making longer expositions, you will "spill" some initially invisible objects into whole pixels, but it is not fair to count pixels - these objects are geometrically much smaller than the pixel in question.

Galaxies (including, but not limited to, the Milky way) represent themselves as an object with size and shape - again - only because of our limited resolution. Otherwise, they are quite sparse and transparent. Stars themselves have quite a lot of surface brightness, but a whole galaxy is rather faint because it averages "a little" of stars and many orders of magnitude more dark empty space.