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Dark energy is probably the most mysterious idea in all of physics right now. It’s a good deal more mysterious than dark matter, a substance which is itself hidden in the shadows. Dark matter, we can at least map, and understand it as physically distributed in the universe, much like regular matter. But dark energy is so poorly understood that the term is barely even defined. What is dark energy? Dark energy is anything that makes certain enormous flaws in modern cosmology make sense.

After Einstein, physicists had been fairly sure how the universe was evolving: the Big Bang threw all the matter outward, as an explosion does, and now gravity is slowing that outward expansion through the natural tendency of matter to not want to move away from other matter. Einstein said that the universe was static, but later accepted evidence produced by Edwin Hubble that the universe was in fact expanding, and calling the static universe hypothesis his “greatest blunder.”

Admit it, scientists: this doesn’t actually do anything.

Yet, surely, even an expanding universe must still make some sense. It was unclear whether the universe would ever fully stop expanding, or perhaps begin to contract back in on itself resulting an an eventual Big Crunch. Still, one thing was for sure: the universe sure wasn’t getting bigger faster.

Except it is. Studies looking at Type Ia supernovae, so-called “standard candles” in astronomy, provided the first ever evidence that the expansion of the universe is not occurring more and more slowly, but more and more quickly as time goes on. Arriving in 1998, this was one of the biggest scientific announcements of the century. Later, evidence from things like maps of the Cosmic Microwave Background radiation showed that this was correct: in defiance of the gravitational influence of matter, the universe is expanding ever faster.

More than that, it seemed that there was simply more stuff in the universe than had been known. The previously mentioned CMB maps can provide a value called the critical density, which given certain observations ought to equal the total amount of matter and energy in the universe. The actual observable amount doesn’t remotely equal this figure, however, and even with dark matter taken into account the universe still only seems to have about a third as much stuff in it as expected.

Physicists realized that these two observations can have identical explanations: there is a whole lot more energy in the universe than we had previously realized, and that energy could be driving the expansion we observe in the universe.

Crucially, one of the other names you might run across for dark energy is “the energy of space.” Einstein referred to this as the cosmological constant, but he didn’t have a good understanding of what it was he was referring to (part of why it burned him so badly). This would make dark energy an intrinsic property of space — if you have some dark energy in some space, and then you expand that space by 100%, you don’t dilute the dark energy you had because the new space you just created will have dark energy all its own. We know that, because all space has dark energy, or else it wouldn’t be space, by this theory’s definition of the term.

So, as the universe expands, this energy can keep driving that expansion, since it doesn’t get diluted. If it did, like regular matter and energy, its influence would be smeared out to the point that gravity dominated and expansion would slow or stop.

The big problem with this idea is that we can calculate what the value of the cosmological constant should be — that is, the strength of the hypothetical repulsive force of dark energy — and that calculation leads to figures much higher than we actually observe. If the cosmological constant were what it is supposed to be, the universe would be flying apart at a much faster rate than we actually observe, unless some other unknown force is counteracting it.

On the other hand, a competing theory of dark energy says that it might be a quantum field that fills space up, forcing it expand through simple fullness. Rather than more space creating more dark energy, in this conception of the universe, thing happen the other way around.

However, this idea should imply that the orbits of planets and other macro-scale objects would be disturbed by dark energy — but they don’t seem to be. Some physicists have posited a so-called “chameleon” field for dark energy, which is dispersed into nothingness when it comes into contact with very dense or massive objects. In this case, “very dense” would refer to anything larger than just a few atoms.

You’ll notice that there is a lot of speculation in this field, and a lot of wishful thinking. How do we know that dark energy isn’t another of Einstein’s cosmological constants, a fictional concept invented to shore up a failing theory? Why couldn’t dark energy be just another foolhardy attempt to explain away inconvenient observations?

A series of maps of the universe, concentrically enclosing the Earth.

Because of the Cosmic Microwave Background radiation — you forgot about that, didn’t you? Scientists may not know much about dark energy, but they know that it almost certainly exists, due to their ability to account for the total amount of matter and energy of all types. This energy, the majority of the universe, is by far the most likely candidate to be causing the quickening expansion of the universe. Calculations have put the certainty of the existence of something much like dark energy at 99.996%.

But make no mistake: dark energy is one of the least-understood concepts ever to be widely accepted by science. Understanding dark energy would not only be philosophically important, but it would give scientists a much more accurate idea of how the universe came into being, and evolved after that point.

The Dark Energy Survey and other universe-level studies have a lot to offer to the future of dark energy research, but the the Large Hadron Collider, recently reopened after significant power upgrades, could provide a totally new form of evidence on the matter. By smashing atoms together so violently, they may finally be able to measure dark energy in a bottle.

Check out our ExtremeTech Explains series for more in-depth coverage.