Outer Space is comprised of the relatively empty regions of the universe out side the atmospheres of the celestial bodies (for instance, Earth, or the other planets in our solar system). Outer space is used to distinguish from airspace and terrestrial locations, it is not completely empty but contains low density particles as well as dark energy and dark matter.
Space is the closest form to a perfect vacuum that has been found. It has no friction, which allows all of the celestial objects within it to move along at whichever gravitational trajectory it is a part of.
The Celestial objects keep their atmospheres by adhering to their gravitational attraction. Every gravitational field has a different density. For instant, the Earth's atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 1 Pa (10-3 Torr) at 100 km of altitude, which is a common definition of boundary in the outer space. The observable universe is filled with photons which are the cosmic background radiation which has an equally correspondingly number of neutrinos.
The atmospheric density within outer space produces drag on satellites. Most artificial satellites stay in low Earth orbit to deal with this drag, and fire their engines every few days to maintain orbit. Planets are too massive to be affected by the drag of the Solar System.
A human exposed to the vacuum of space would not die from exposure, but would rather quickly asphyxiate due to the intense pressure and lack of air in outer space. Once the deoxygenated blood reached the brain, death would quickly follow. Cold or oxygen rich atmospheres can survive at as long as the oxygen density is similar to that of a standard-sea level atmosphere.
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