Recent discoveries in string theory have allowed for dark energy to exist within string theory. Even more intriguing than dark matter is dark energy, which is a positive energy that seems to permeate the entire universe and to be much more abundant than either form of matter — but also much less abundant than physicists think it should be.
Although string theory offers some possibilities for dark matter, it offers less explanation for dark energy. Theoretically, dark energy should be explained by the value of the vacuum energy in particle physics, where particles are continually created and destroyed. These quantum fluctuations grow immensely, leading to infinite values.
However, when physicists try to use their standard methods to compute the value of the vacuum energy, they get a value that is off from the experimental value of dark energy by 10120!
The real value is incredibly small, but not quite zero. Though the amount of dark energy in the universe is vast (according to recent data, it makes up about 73 percent of the universe), the intensity of dark energy is very small — so small that until 1998, scientists assumed the value was exactly zero.
The existence of dark energy (or a positive cosmological constant, depending on how you want to look at it) doesn’t remove the many solutions of string theory relating to different possible physical laws. The number of solutions that include dark energy may be on the order of 10500. This dark energy reflects a positive energy built into the very fabric of the universe, likely related to the energy of the vacuum itself.
To some, the ekpyrotic universe has a benefit over the inflationary model, because it offers a reason for why we might observe such a value for dark energy in our universe: That’s the part of the cyclic phase that we’re in.
At times in the past, the dark energy may have been stronger, and at times in the future it may be less. To many others, this reason isn’t any more intellectually satisfying than the lack of a reason in other cosmological models. It still amounts to an accidental coincidence (or an application of the anthropic principle).
Outside of the ekpyrotic universe, there’s little explanation for what’s going on. The problem of offsetting the expected vacuum energy by such a large amount — enough to almost, but not quite, cancel it out — is seen by many physicists as too much chance to contemplate.