What is Dark Energy?
The further we look into the cosmos, the less sense it makes. That's the feeling of scientists now struggling with the problem of dark energy. This unknown force is the dominant stuff of the universe, and at this point it is a big mystery.
There are several theories being proposed to explain dark energy. So far testing these ideas has been very hard to do. To do this, they need new scientific instruments to search deeper into the universe.
Right now, the only way to talk about dark energy is to say what we know it does. It creates more space, new space by pushing galaxies further apart. This makes the entire universe grow larger at a faster rate. In the late 1990s, studies of distant supernovae showed space was expanding faster than expected.
There was one big hint that dark energy existed even before it was discovered by astronomers. The great Albert Einstein had included an "anti-gravity" effect called the Cosmological Constant in his Theory of general relativity to make it work correctly. After Edwin Hubble discovered the universe was expanding, Einstein and other scientists viewed this as an annoying "fudge factor" that had no connection to the real universe.
Later researchers described the Cosmological Constant as an underlying background energy. That energy might exert some kind of pressure on the cosmos. Unfortunately, the theory predicts the energy ought to be much stronger than dark energy appears to be.
Another idea for dark energy is something called quintessence. The word is the same as the ancient Greek term for a mysterious fifth element beyond earth, air, fire and water. Unlike the cosmological constant, the modern theory of quintessence is an energy field that pushes particles apart. It can also lessen over space and time. This is handy because observations suggest that dark energy has only been in effect for about the last 5 billion years – so its effect is not constant.
Scientists need to learn much more about dark energy's impact on the universe to test their theories. The only way to do that, of course, is with more data from the universe.