The vacuum is the space which is not filled with anything. In it there is neither energy, nor masses. It is the emptiness deprived of matter. In modern physics these criteria are a little corrected. There are two types of a vacuum: technical and physical, these concepts differ a little.
The concept about a vacuum changed over time. At the very beginning of development of sciences about the world around the vacuum was meant just as emptiness, even the word vacuum is transferred from Latin as "emptiness". It was rather philosophical category as to investigate something, at least remotely meeting expectations of a vacuum, scientists had no opportunity. The modern physics calls a vacuum such condition of the quantum field at which its power state is at the lowest level. This state is characterized first of all by the fact that real particles in it are absent. A technical vacuum call strongly rarefied gas. It is not absolutely ideal vacuum, but the fact is that in actual practice it is unattainable. All materials pass gases in microscopic volumes therefore any vacuum concluded in a vessel will have hindrances. Degree of its sparseness is measured by means of parameter λ (lambda) which specifies length of a free run of a particle. This distance which it can pass until faces an obstacle in the form of other particle or a wall of a vessel. A high vacuum – such at which I can pass molecules of gas from one wall to another, practically never facing with each other. The low vacuum is characterized by rather large number of collisions. But even if to assume that it will be possible to reach an ideal vacuum, then all the same you should not forget about such factor as thermal radiation – so-called gas of photons. Thanks to this phenomenon temperature of body placed in a vacuum would become the same as vessel walls after a while. It will occur exactly thanks to the movement of thermal photons. The physical vacuum is a space in which weight is absent completely. But, according to the quantum theory of the field, even at such state it is impossible to call it absolute emptiness as in a physical vacuum continuously there is an education and disappearance of virtual particles. They are called still zero fluctuations of the field. There are various theories of the field according to which properties of bezmassovy space can vary a little. It is assumed that the vacuum can be one of several types, the features are inherent in each of which. Some of those properties of the quantum field in a vacuum which were predicted by scientists-theorists were already confirmed experimentally. Is among hypotheses and such which check will be able to confirm or refute fundamental theories of physics. For example, the assumption that so-called false vacuums (various vacuum states) are possible is very important for confirmation of the inflationary theory of the Big Bang.