The detection of what is now called “dark energy” is one of the major science discoveries. This discovery is due to the power of the Hubble telescope which gives astronomers their best ever views of the universe.
The gravitational attraction of all the matter in the universe should cause cosmic expansion to slow down. But in 1998, two groups of astronomers discovered the exact opposite: The rate of universal expansion is accelerating. The researchers based their discovery on observations of stellar explosions known a type Ia supernovae, which occur when white dwarf stars grow to their limiting mass of about 1.4 solar masses. Only the Hubble telescope could view the most distant of these explosions and thus confirm the acceleration.
All current studies indicate that a still-mysterious form of energy, dubbed dark energy, propels this speed-up. Although scientists do not yet understand the precise nature of dark energy, they have deduced some of its properties. These efforts suggest that it is the energy associated with empty space, or what scientists call the physical vacuum.
That the vacuum contains energy is not surprising in itself. Quantum mechanics—the physics that describes the universe at the smallest scales—predicts that the physical vacuum is far from empty. Instead, it teems with virtual pairs of particles and antiparticles that appear and disappear within tiny fractions of a second. The problem has been that every theoretical attempt to calculate what the energy density of the vacuum should be has missed the target by several orders of magnitude.
Given the quickening expansion, what will the fate of our universe look like in the distant future? If dark energy does represent the energy of empty space, which as a constant density, then the expansion will continue to accelerate. About a trillion years from now, astronomers living in the merged product of the Milky Way and the Andromeda Galaxy—the two are expected to collide about 4 billion years from now —will not be able to see any other galaxy. The universe then will be well on its way toward a cold death.
 See Mario Livio, “Top Seven Science Discoveries,” Astronomy (43, 4, April 2015), pp. 28-35