In recent years, accumulating evidence supports the fact that besides dark matter, the universe is rich with dark energy. It primarily shows measurements of distant supernova's which clearly indicate that the expansion of the universe accelerating, not slowing down as it was thought until recently. Since all common substances act gravitationally attractive and slow the expansion of the universe, this acceleration indicates on new ingredient in the universe that works in the opposite direction and forcing it to spread rapidly. That ingredient is called dark energy. What it is really and what's its origin is still unknown, but there are several possibilities. The most famous of these is that the so-called dark energy, is a cosmological constant, which was long ago introduced into A. Einstein equations (but he rejected this constant).
Einstein believed that the universe is static, so he introduced the constant into his equations, repulsive force that balances the gravitational attraction among the galaxies. When Hubble later discovered that the universe is expanding, Einstein threw out the constant from the equation and it was designated as "his greatest mistake". 1998th scientists have discovered that the universe is expanding rapidly under the influence of the repulsive force that stretches space, the mysterious force has become known as the concept of dark energy. Theorists then revived Einstein's cosmological constant. According to general relativity, the cosmological constant has negative pressure, it follows that empty space has a positive energy, and that the cosmological constant increases the speed of expansion of empty space.
Dark energy is the name for a hypothetical form of energy that penetrates in the whole area and has a negative pressure that results in a repulsive gravitational force. Dark energy works opposite of ordinary matter which through gravity acts attractive and thus slows the expansion of the universe.