When the New Horizons probe reached the outer dark of the Solar System, out past Pluto, its instruments picked up something strange.
Very, very faintly, the space between the stars was glowing with optical light. This in itself was not unexpected; this light is called the cosmic optical background, a faint luminescence from all the light sources in the Universe outside our galaxy.
The strange part was the amount of light. There was significantly more than scientists thought there should be – twice as much, in fact.
Now, in a new paper, scientists lay out a possible explanation for the optical light excess: a by-product of an otherwise undetectable interaction of dark matter.
"The results of this work," write a team of researchers led by astrophysicist José Luis Bernal of Johns Hopkins University, "provide a potential explanation for the cosmic optical background excess that is allowed by independent observational constraints, and that may answer one of the most long-standing unknowns in cosmology: the nature of dark matter."
We have many questions about the Universe, but dark matter is among the most vexing. It's the name we give to a mysterious mass in the Universe responsible for providing far more gravity in concentrated spots than there ought to be.
Galaxies, for instance, rotate faster than they should under the gravity generated by the mass of visible matter.
The curvature of space-time around massive objects is greater than it should be if we calculated the warping of space based only on the amount of glowing material.
But whatever it is creating this effect, we can't detect it directly. The only way we know it's there is that we just can't account for this extra gravity.
And there's a lot of it. Roughly 80 percent of the matter in the Universe is dark matter.
There are some hypotheses about what it might be. One of the candidates is the axion, which belongs to a hypothetical class of particles first conceptualized in the 1970s to resolve the question of why strong atomic forces follow something called charge-parity symmetry when most models say they don't need to.
As it turns out, axions in a specific mass range should also behave exactly like we expect dark matter to. And there might be a way to detect them – because theoretically, axions are expected to decay into pairs of photons in the presence of a strong magnetic field.
Several experiments are searching for sources of these photons, but they should also be streaming through space in excess numbers.
The difficulty is in separating them from all the other sources of light in the Universe, and this is where the cosmic optical background comes in.
The background is itself very difficult to detect since it's so faint. The Long Range Reconnaissance Imager (LORRI) aboard the New Horizons is possibly the best tool for the job yet. It's far from Earth and the Sun, and LORRI is far more sensitive than instruments attached to the more distant Voyager probes that launched 40 years earlier.
Scientists have presumed the excess detected by New Horizons to be the product attributed to stars and galaxies that we can't see. And that option is still very much on the table. The work of Bernal and his team was to assess whether axion-like dark matter could possibly be responsible for the extra light.
They conducted mathematical modeling and determined that axions with masses between 8 and 20 electronvolts could produce the observed signal under certain conditions.
That's incredibly light for a particle, which tends to be measured in megaelectronvolts. But with recent estimates putting the hypothetical piece of matter at a fraction of a single electronvolt, these numbers would demand axions to be relatively beefy.
It's impossible to tell which explanation is correct based solely on the current data. However, by narrowing down the masses of the axions that could be responsible for the excess, the researchers have laid the foundations for future searches for these enigmatic particles.
"If the excess arises from dark-matter decay to a photon line, there will be a significant signal in forthcoming line-intensity mapping measurements," the researchers write.
"Moreover, the ultraviolet instrument aboard New Horizons (which will have better sensitivity and probe a different range of the spectrum) and future studies of very high-energy gamma-ray attenuation will also test this hypothesis and expand the search for dark matter to a wider range of frequencies."
The research has been published in Physical Review Letters.