The solution to one of the deepest and most intractable mysteries facing modern physics might be found in one of Albert Einstein’s forgotten theories that the famous physicist abandoned nearly a century ago.

Dark energy and dark matter are invisible theoretical substances that are thought to make up 95 percent of the universe, but their existence is only theorized based on the effects they appear to have on the normal matter we’re all familiar with. Some of the most sophisticated and sensitive instruments ever made by humans have failed to detect any sign of the stuff after nearly 50 years of searching.

As detailed in a paper published this week in Astronomy and Astrophysics , a theory developed by Albert Einstein in 1918 and then abandoned may have held the key to the mystery of dark matter and dark energy all along. Oxford astrophysicist James Farnes drew on this theory to come up with a new theory that unifies dark matter and dark energy as a single “dark fluid” that permeates the universe.

This dark fluid, if it exists, has negative mass. Unlike normal matter, which has a positive gravitational charge or mass (meaning it attracts other matter), negative mass would repel matter. In short, if you pushed an object that had negative mass away from you, the object would actually move toward you rather than moving in the direction of the applied force, as is the case with ordinary matter. According to Farnes, negative masses would be spread throughout the universe as a single substance in the form of dark fluid.

“The outcome seems rather beautiful,” Farnes said in an article about his theory at The Conversation. “Dark energy and dark matter can be unified into a single substance, with both effects being simply explainable as positive mass matter surfing on a sea of negative masses.”

Farnes’ new theory is both elegant and intuitive. After all, as Farnes points out in his paper, polarization—simply, things existing in positive and negative forms—is a common property in the universe. There are positive and negative electrical charges, and even information itself appears to be polarized as ones and zeroes. It would be odd, Farnes argues, if such a fundamental property as mass has monopolized positive charges.

Farnes’ theory has its roots in a small note Einstein made to himself in 1918 while struggling to explain the cosmological constant—which Einstein first used to describe the dynamics of the universe—in his equations for general relativity.

Einstein invoked the cosmological constant to explain how the universe could be static, which was widely accepted at the time, while also accounting for the effects of gravity. Without this cosmological constant, Einstein realized, the gravitational force of the universe would cause it to collapse upon itself. Basically, the cosmological constant was a term that functioned as a sort of anti-gravity. The problem for Einstein was to explain what this cosmological constant consisted of.

In the 1918 note, Einstein described a modification of his theory of general relativity in which “‘empty space’ takes the role of gravitating negative masses which are distributed all over the interstellar space,” the key phrase here being “negative mass.”

By the following year, however, Einstein had adopted a different interpretation of the cosmological constant and this small note was lost to history. In 1931 Einstein removed the cosmological constant from his theory of general relativity entirely after Edward Hubble discovered that the universe was not static, but expanding.

This crushing observational evidence led Einstein to describe his invocation of the cosmological constant as his “greatest blunder.” Today, however, Einstein’s cosmological constant is hardly considered a blunder by most physicists.

In fact, the cosmological constant is integral to the the lambda-CDM model, the most widely accepted cosmological model of the universe. In this model of the universe the cosmological constant represents dark energy, which is invoked to explain the accelerating expansion of universe.

The lambda-CDM model also incorporates dark matter as a way to explain observed galactic rotation. The gravitational influence on the stars on the outskirts of a galaxy is less than the gravitational influence on the stars at the center of the galaxy, which suggests that the stars on the outskirts of the galaxy should be rotating faster than the inner stars. In fact, galaxies should be flying apart due to their own rotational force. Dark matter is theorized as the stuff that keeps galaxies intact and accounts for the observed rotational speed of stars.

Yet in the past half century, dozens of experiments meant to detect dark matter have come away empty handed.

In the case of dark matter, a large part of the problem is that physicists aren’t exactly sure what they’re looking for since there are a number of leading candidates for dark matter particles. As for dark energy, which is thought to be a property of space itself, there are also a number of different theories that range from virtual particles that pop in and out of existence to a type of field known as “quintessence.” In either case, physicists have no idea how to detect dark energy and can only posit its existence based on the expansion of the universe.

There is, of course, the possibility that Einstein got gravity totally wrong and we need to abandon the concept of dark matter and dark energy entirely. While some physicists have created alternative theories of gravity that obviate the need for dark matter, these are generally considered fringe in the scientific community. This is mostly because they usually require altering Einstein’s theory of general relativity, which has been proven correct in every test thrown at it in the past century.

Farnes’ new theory is a radical departure, but if it’s correct it would fundamentally change our understanding of the universe.

So far, the first rudimentary models Farnes created based on his theory have been able to account for a handful of observed properties of galactic rotation and the expansion of the universe that are normally attributed to dark matter and dark energy. This is a promising start, but Farnes said more observational data from instruments like the Square Kilometer Array will be needed to push his theory into the mainstream.