Casimir pressure is created by the Casimir force of virtual particles.
According to experiments, the Casimir force between two closely spaced neutral parallel plate conductors is directly proportional to their surface area :
Therefore, dividing the magnitude of Casimir force by the area of each conductor, Casimir pressure can be found. Because the Casimir force between conductors is attractive, the Casimir pressure in space between the conductors is negative.
Because virtual particles are physical representations of the zero point energy of physical vacuum, the Casimir pressure is the difference in the density of the zero point energy of empty space inside and outside of cavity made by conductive plates.
Some scientists believe that zero point energy is the dominant energy of the Universe and that the Casimir pressure of this energy is the main cause of the observed accelerated expansion of the Universe. In other words, virtual particles drive the accelerated expansion of the Universe.