The Standard Model of particle physics is currently our best understanding of how the universe works – but it only describes about five percent of everything in it. The rest is made up of what we call dark matter and dark energy, which are so far only known through their gravitational interactions with regular matter. Now, an astrophysicist from Oxford has put forward a new theory that suggests that dark matter and dark energy are actually part of the same phenomenon: a "dark fluid" with negative mass that fills the universe.

In a way, dark matter and dark energy are both placeholder concepts, plugging holes between the Standard Model and what we actually observe. For instance, the observed movement and distribution of galaxies doesn't make sense if their mass is limited to the stuff we can see. Since the 1930s, this hidden extra mass has been dubbed dark matter.

Dark energy is a more recent concept. The observation that the expansion of the universe seems to be accelerating was only made in 1998, when it was discovered that more distant objects are moving away from us faster than those closer by. The mysterious force that drives this, which we still know very little about, is now referred to as dark energy.

Taken together, dark matter and dark energy form the basis of our current standard model of Big Bang cosmology, the Lambda-CDM model. The Lambda in that name denotes dark energy as a kind of cosmological constant, while CDM stands for "cold dark matter," which seems to be the most accurate theory of the stuff – it's "cold" because it moves relatively slowly and interacts fairly weakly with ordinary matter.

Dark matter and dark energy have always been treated as separate entities, but are they in fact two sides of the same coin? That's the core idea behind the new theory put forward by Oxford astrophysicist Jamie Farnes, which may expand on the Lambda-CDM model.

Dark fluid theory

Farnes' new theory says that 95 percent of the cosmos is made up of a "dark fluid," and dark matter and dark energy are effectively both "symptoms" of that underlying phenomenon. It does do a good job of describing both of those, although it requires a little number-fudging of its own.

This dark fluid would need to have negative mass. That alone sounds like a sci-fi concept – how can something have a mass of -1 kg? But according to Newtonian physics it's entirely possible, albeit still hypothetical.

Something that has negative mass would have some pretty weird characteristics. For one, forces are inverted, so if you were to push a ball with negative mass it would accelerate towards your hand, instead of away from it. That also means it exhibits a kind of negative gravity, which repels other material instead of attracting it.

If the cosmos is filled with dark fluid, its negative gravity would be pushing everything away from everything else – exactly the observed phenomenon that dark energy was invented to explain. Meanwhile, it's not the gravitational pull of a dark matter halo that's holding galaxies together – it's the negative "push" of the dark fluid surrounding them. Galaxies of regular matter are basically bubbles floating in a cosmological dark fluid.

Do negative masses even exist?

One of the main issues with the theory is that we don't yet know if negative masses exist. But, Farnes argues in the study, other physical forces all seem to be polarized, so why wouldn't mass also be positive and negative?

"For example, electric charges (+ and −), magnetic charges (N and S), and even quantum information (0 and 1) all appear to be fundamentally polarized phenomena," the paper reads. "It could therefore be perceived as odd that gravitational charges – conventionally called masses – appear to only consist of positive monopoles."

Lending more weight to the idea, other recent studies have managed to create fluids and particles that exhibit the properties of negative mass.

A matter of creation

Another problem with the theory is that as the universe expands, the dark fluid would thin out to the point that its effects are no longer seen. To counter that, Farnes introduces a "creation tensor" into his equations, which essentially suggests that this negative mass matter is constantly being created anew, keeping the dark fluid at a regular consistency over time.

That might sound a little too convenient, but the idea of matter creation has some precedent. It was part of the Steady State model, an early alternative to the Big Bang theory that has since been disproven observationally. But it was dealing with regular (or positive mass) matter – Farnes says negative mass matter might still behave that way.

"Previous approaches to combining dark energy and dark matter have attempted to modify Einstein's theory of general relativity, which has turned out to be incredibly challenging," says Farnes. "This new approach takes two old ideas that are known to be compatible with Einstein's theory – negative masses and matter creation – and combines them together. The outcome seems rather beautiful: 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."

If true, the dark fluid theory could explain why the ongoing hunt for dark matter particles has consistently come up empty-handed. Of course, there's every chance that Farnes is wrong, and the astrophysicist acknowledges that, but he hopes that it will at least get the scientific community discussing different ideas. To find evidence to back up the idea, Farnes plans to use the Square Kilometer Array (SKA) telescope to compare observations of the universe to predictions made by the dark fluid theory, including looking for signs that negative masses exist in the universe.

"There are still many theoretical issues and computational simulations to work through, and Lambda-CDM has a nearly 30 year head start, but I'm looking forward to seeing whether this new extended version of Lambda-CDM can accurately match other observational evidence of our cosmology," says Farnes. "If real, it would suggest that the missing 95 percent of the cosmos had an aesthetic solution: we had forgotten to include a simple minus sign."

The study was published in the journal Astronomy and Astrophysics.