Dark energy is arguably the best mystery in astrophysics. Here we have an uneasy placeholder for almost all of the energy in the universe—energy that, as you read this, is working hard to shred the universe itself. Energy that will not be satisfied until all of existence is a featureless black void. Dark energy also has the bonus selling point of being a fairly new idea, tracing back to the late-1990s discovery that the universe is not just expanding, but is also accelerating in its expansion. At every moment, the universe gets both bigger and emptier (more space, same amount of stuff).
What's doing this or, better, why it's happening is unsettled, to put it mildly. The general answer is that quantum physics insists that empty space has energy—vacuum energy—but what, exactly, that means is TBD. In a paper posted this week to the arXiv pre-print server, a group of cosmologists from the University of Barcelona make an interesting case for dark energy being linked to so-called frozen neutrinos, or neutrinos that may have become coupled to dark energy as the universe began to cool circa a million years following the Big Bang. These neutrinos, which had before been hauling ass around the universe at near-light speed, were suddenly arrested, passing on their kinetic energy (energy of motion) to the dark energy field in the process.
Dark energy is a big mystery, but it may be easier to conceptualize than it's often made out to be. Simply, empty space has energy. It must have energy, per quantum mechanics. Truly zero energy is forbidden because "absolute nothing" is far too certain of a state given physics that forbid us from knowing completely and simultaneously both the position and momentum of a particle. "No particle" breaks this rule because it says something with 100 percent certainty about both a particle's position and momentum; namely, that it has neither. This implies that empty space must allow some non-zero probability of not being empty, which implies that particles will in fact appear from nothing because the alternative is not physically possible.
This is the idea behind vacuum energy and it's something that we can actually observe as the Casimir effect. Empty space is constantly giving birth to pairs of new particles called virtual particles, which usually just annihilate when they crash into other particles. This resulting fizz of short-lived particles is the leading explanation for dark energy. As the universe expands, the energy of empty space—which is very, very, very small—becomes the dominant force and acts as an outward pressure, shoving the universe apart. The more the universe is shoved apart, the more empty space exists. Thus, the expansion accelerates.
There is a great big catch, however. The expected vacuum energy offered by the above process is much, much
smaller larger than what is required to drive the observed expansion of the universe. This opens the door to a second, alternative explanation, which is known as quintessence. Quintessence suggests that there is some vast, irregular field of something yet to be discovered. Crucially, quintessence is allowed to change through time, unlike vacuum energy, which is always just there and has always just been there. Quintessence, however, has to be very finely tuned for cosmic acceleration to work out, which is a problem with the theory.
However, according to the new paper, cold neutrinos offer a way out. It turns out that if you couple this dark energy quintessence to some other field, that quintessence will suddenly start behaving as required. As Ethan Siegel notes at Starts With a Bang, this theory may even be testable. "There are experimental signatures that we could look for to tell this mechanism apart from all others," he writes. "There's a specific type of decay that's possible in some subatomic particles: neutrinoless double beta decay, where an atomic nucleus emits two electrons and no neutrinos. This is a decay that has never been seen before, but if this model is correct, not only will it be real, it will have a distinct signature from all the other models out there."
I'm not sure any quintessence theory is going to have the gut sense of vacuum energy/virtual particles, but we are after all constrained by data. So, for the time being it remains open season for dark energy theorizing.