Dark energy is a negative-pressure component of the Universe whose negative pressure is postulated to account for the accelerated expansion of the Universe, first detected by two teams of astrophysicists in 1998. Although its nature is not well understood, it comprises the bulk (about 70%) of the mass-energy of the Universe. Albert Einstein originally suggested a form of dark energy might exist (which he called the cosmological constant) about 80 years ago, but theorists have developed a variety of intriguing alternative ideas. Today, understanding the nature of dark energy is one of the biggest challenges in physics.
Dark matter is a mysterious substance that neither emits nor absorbs light but is required to account for the gravitational fields that hold galaxies together. By measuring the gravitational effects of dark matter in a variety of astrophysical and cosmological settings, we now know that dark matter outweighs the ordinary atomic matter of which we are composed by roughly a factor of 6. Still, the nature of this dark matter remains a complete mystery. There are good reason, though, to postulate that this dark matter consists of some new, yet undiscovered, elementary particle, and particle theorists have been prolific in producing detailed models for such particles. This theoretical work is paralleled now by vast experimental efforts to infer directly the nature of this dark matter.
The Early Universe
Around 1980, a notion emerged in theoretical cosmology that the cosmological expansion may have been set in motion by “inflation,” a brief period of superluminal expansion in the very early Universe. Although speculative, observations and measurements done in the decades since 1980 have verified a number of the predictions of inflation. In order to work, though, inflation requires some new fundamental physics, beyond Einstein’s theory of gravity and the strong, weak, and electromagnetic forces that govern the behavior of sub-atomic matter. The search to understand how inflation worked is now one of the most exciting endeavors in physics.