Quanta: It is the so-called “dark energy.” By Frank Wilczek
'Dark energy is essentially — up to a numerical factor — the same thing Einstein called the “cosmological constant.” If you average it over the entire universe, you find that dark energy contributes about 70 percent of the total mass in the universe.
'That sounds impressive, but for physicists the big puzzle that remains is why its density is as small as it is. For one thing, you’ll remember, it was supposed to be infinite, due to the contribution of fluctuating fields. One bit of possible progress is that now we know a way to escape that infinity. It turns out that for one class of fields — technically, the fields associated with particles called bosons — the energy density is positive infinity, while for another class of fields — those associated with particles called fermions — the energy density is negative infinity. So if the universe contains an artfully balanced mix of bosons and fermions, the infinities can cancel. Supersymmetric theories, which also have several other attractive features, achieve that cancellation.
'Another thing we’ve learned is that in addition to fluctuating fields, the vacuum contains non-fluctuating fields, often called “condensates.” One such condensate is the so-called sigma condensate; another is the Higgs condensate. Those two are firmly established; there may be many others yet to be discovered. If you want to think of a familiar analogue, imagine Earth’s magnetic or gravitational field, elevated to cosmic proportions (and freed of Earth). These condensates should also weigh something. Indeed, simple estimates of their density give values far larger than that of the observed dark energy.
'We’re left with an estimate of the dark energy that is finite (maybe), but poorly determined theoretically and, on the face of it, much too big. Presumably there are additional cancellations we don’t know about. The most popular idea, at present, is that the smallness of the dark energy is a kind of rare accident, which happens to occur in our particular corner of the multiverse. Though unlikely a priori, it is necessary for our existence, and therefore what we are fated to observe.'