Ostensibly one might think that this article is about nothing - since a vacuum can be considered an absence of matter. However, modern physics suggests that the vacuum may be a lot more interesting than this. In modern physics the vacuum is a sea of virtual positron electron pairs, continually coming into existence and annihilating. Sometimes, when these virtual pairs come across energetic particles they can interact with them and produce real particles at the expense of the third body.
In the astronomical context, even in the emptiest corners of the Universe, the vacuum is not perfect. It contains on the order of one atom or ion every cubic metre. However, modern physics suggests that even this is not the complete picture, as atomic matter only makes up a few percent of the total mass of the Universe. The two other major contributors to the mass are dark matter and dark energy. Although the effects of these mysterious components can only be observed indirectly (through their influence on galactic and cosmological scales) it must be assumed that they too are present even in the remotest corners of the Universe. Dark energy in particular is thought to pervade all of space, and is actually described as a property (energy density) of the vacuum itself. This is the physical manifestation of the cosmological constant first introduced by Einstein.