There are few mysteries in the universe more confounding than that of dark matter and dark energy. In fact, most cosmologists don’t even like those terms because they imply a property—darkness—to forces which scientists can’t observe. All humans can currently observe of dark matter is the profound gravitational influence it exerts on surrounding matter.

All we can observe of dark energy is that it is accelerating the rate at which the universe expands, so much so that someday in the distant future galaxies will be completely isolated from each other. Future scientists may think that our Milky Way and its local galaxy cluster are the only galaxies in the entire universe.

Combined dark matter and dark energy occupy 96% of the universe yet scientists have no idea what these forces are and without them, humans likely would not exist. If it weren’t for dark matter, galaxies wouldn’t exist because none of the matter would coalesce. Without dark energy unfolding at the exact rate it did, it’s unlikely the evolution of the universe would have supported life.

Theories about the true nature of dark energy run the spectrum of scientific ideology, ranging from a fifth fundamental force (in addition to gravity, electromagnetism, weak force, and strong force) and exotic anti-gravitational particles to a “quintessent” energy field that arises during different epochs of the universe and “tampers” with the laws of physics.

This theory of “quintessence” received an extra boost from a new hypothesis that posits that when the universe was only 100,000 years-old, an “early dark energy” field formed, accelerating the early universe and then disappearing another 100,000 years later. The new theory seeks to try to explain the 9% discrepancy in the expansion of the universe with the idea of periodic fields of dark energy.

Dennis Overbye, part of a team of astronomers from Johns Hopkins University, described this energy force as “a buzzing, expanding mass of particles and radiation—a strange new energy field switched on. That energy suffused space with a kind of cosmic antigravity, delivering a not-so-gentle boost to the expansion of the universe.”

This theory would explain a third infusion of a nebulous energy force in the universe. The first happened soon after the Big Bang, when the universe was less than a trillionth of a second old and a tiny micro-second of “inflation” violently expanded the universe and allowed for order in the cosmos. The second known period of a mysterious propulsive force in the universe is right now. Galaxies are accelerating away from each other faster and faster—and scientists don’t know why.

The debate over dark energy has raged within the scientific community but astronomers are still no closer to explaining it. That hasn’t stopped a proliferation of theories to account for it: non-baryonic sub-atomic particles destroying themselves and releasing energy; a manifestation  of a pervasive symmetron field similar to the Higgs Field; a universal “dark fluid” that combines both dark matter and dark energy and represents “negative gravity”; evidence of the exotic energy fields predicted by “String Theory”; and even evidence of a multiverse.

Theories of the multiverse posit that a near infinite number of universes are simultaneously evolving. Perhaps in the majority of these, dark energy accelerates the fabric of space-time too quickly or not enough and the universe is destroyed and we just happen to exist in the one anthropically attuned to life.

Essentially, there are two camps: those who believe dark energy represents a cosmological constant and those that don’t. Previously, the cosmological constant theory—an unexpected throwback to what Einstein referred to as his “biggest blunder”—had the most support, even if the math didn’t exactly support it.

Newer fringe theories have even suggested that advanced extraterrestrial life could be involved. Some astronomers consider whether dark matter and dark energy may constitute “a living state that manipulates luminous matter for its own purposes.” In a stunning article, Caleb Scharf, a research scientist at Columbia University and director of the Columbia Astrobiology Center, postulates the idea that certain laws of nature and physics may be embedded with alien intelligence.

It’s a great mind-bending twist. Perhaps hyper-advanced life isn’t just external. Perhaps it’s already all around. It is embedded in what we perceive to be physics itself, from the root behavior of particles and fields to the phenomena of complexity and emergence,” says Scharf. “What we think might be the effects of mysterious forces such as dark energy and dark matter in the Universe, could actually be the influence of alien intelligence–or maybe even aliens themselves.”

While most astronomers veer away from such exotic, and probably unprovable explanations, it’s certainly the case that a general sense of frustration has set in among scientists who have, for years, looked closely at dark energy. They are no closer to understanding it now than they were when it was discovered in the late 90s.

At what point do we claim the discovery of new physics?” asked one theorist, Josh Frieman.