Clusters of galaxies like this one, MCS J0416.1-2403 located in the constellation of Eridanus, have long been theorized to be bound by cosmological dark matter. Credit: ESA/Hubble, NASA, HST Frontier Fields; Acknowledgement: Mathilde Jauzac (Durham University, UK and Astrophysics & Cosmology Research Unit, South Africa) and Jean-Paul Kneib (École Polytechnique Fédérale de Lausanne, Switzerland)

Tantalizing ‘signals’ from a handful of recent high-energy searches for dark matter are more likely the product of conventional astrophysics than the first tentative detections of the universe’s missing mass, say skeptical astrophysicists.

“A decade ago, no [one] would make these claims without first checking and re-checking that it couldn’t be from some normal astrophysical source,” Stacy McGaugh, an astrophysicist at Case Western Reserve University in Cleveland, told Forbes. “Nowadays, the attitude seems to be that if you don’t immediately recognize what it is, it must be dark matter; [with] no penalty for ‘crying wolf’ over and over again.”

Even so, the theoretical stakes remain high.

That’s because for the better part of a century, cosmological “cold dark matter” has been needed to explain the gravitational dynamics of much of the cosmos’ visible matter; including the rotation rates of massive galaxies like our own.

“By a very large margin, the matter we do see directly in galaxies does not produce enough gravity to hold the galaxies together; dark matter is invoked to provide the extra gravity needed,” Mordehai Milgrom, a physicist at Israel’s Weizmann Institute, told Forbes. That is, Milgrom says, if the standard laws of physics are used to calculate gravity as we know it.

And because non-baryonic (or exotic) dark matter is theorized to only interact with normal matter primarily via gravity, dark matter’s detection has inherently been problematic. Even so, most cosmologists accept the idea that normal dark matter may make up as much as 85 percent of the universe’s missing mass.

The need to invoke dark matter at all stems either from the product of unseen exotic particles that lie well beyond the ken of known physics or is the result of new physics in which gravity behaves differently on the largest scales. Neither scenario is easily tested.

For decades, however, experimental physicists have used both laboratory and astronomical observations from ground and space to search for this missing component.

One of the most recent, as noted this month in the journal Physical Review Letters, involves x-ray emissions from both the Perseus galaxy cluster and the nearby Andromeda galaxy.

Using the European Space Agency’s XMM-Newton telescope, researchers from Switzerland’s EPFL Laboratory of Particle Physics and Cosmology and Leiden University in The Netherlands report that this observed excess of x-ray photons may represent signals of decay by sterile neutrinos. That is, heretofore unverified, hypothetical dark matter particles.

“We have been searching for such a signal since 2005,” Alexey Boyarsky, a professor of physics at Leiden University and the paper’s lead author, told Forbes. “The signal is at the lowest range of experimental sensitivity, and if it were easy to find, we would have found it long ago.”

Boyarsky points out that among the models that are consistent with the dark matter interpretation of this signal, the sterile neutrino is probably the simplest and one of the most natural. Such a particle, he says, can interact with normal matter only via quantum mechanical “mixing” with ordinary neutrinos.

Therefore, says Boyarsky, it is very hard to “catch.”

MIT physicist Paolo Zuccon counters that the sterile neutrino’s existence has also not been proven. “They guess its mass; they guess its properties; and, in particular, how it decays,” said Zuccon told Forbes. “All in all, this claim seems a little weak.”

Or as McGaugh puts it: “Based on those data, I would not claim to have detected anything. This looks like a classic case of the over-interpretation of noisy astronomical data.”

However, Zuccon himself has been involved in searches for this stealthy matter, using a spectrometer mounted on the exterior of the International Space Station (ISS).

Zuccon and colleagues analyzed two and a half years of data from the Alpha Magnetic Spectrometer (AMS), the ISS particle detector that recorded a flux of millions of cosmic rays from all over the galaxy. They found an excess of positrons (antiparticles of electrons) at energies of around 8 gigaelectron-volts (GeV) which the researchers say fits some dark matter models.

“But we are not yet in the position to discriminate between the dark matter hypothesis and an astrophysical source [such as] pulsars,” said Zuccon, who is involved with the AMS search. “Only more data from AMS and/or from other measurements will allow an answer.”

Yet as reported by Nature News earlier this month, ESA’s Planck telescope failed to find the imprint of similar positron excesses in the Cosmic Microwave Background, which logically should have been seen if dark matter particles were also colliding and annihilating at comparable rates in the primordial universe.

McGaugh says in the case of the MIT positron signals, the possible signature of dark matter would correspond to an upper energetic limit on the dark matter particle’s actual decay.

“If they see a [energetic] sharp edge like that which corresponds to a plausible dark matter particle, then I’ll get very interested,” said McGaugh. “Until then, they’ve got nothing that can’t be better understood as astrophysical.”

Searchers have also long invoked our Milky Way’s dense galactic center as a dark matter haven. Earlier this year, researchers used publicly available data gleaned from NASA’s Fermi gamma-ray space telescope to identify an excess of high-energy gamma rays from our galactic center.

The galactic center region has been studied in increasing detail and the case for there being a gamma-ray signal from annihilation of dark matter particles has strengthened considerably, Dan Hooper, an astrophysicist at Fermilab in Batavia, Ill., told Forbes.

Independent verification of dark matter signals, says Hooper, would include detecting gamma-rays from dwarf spheroidal galaxies; excess anti-protons; gamma-rays from nearby dark matter-dominated galactic sub-halos; dark matter particles in underground experiments; or producing them at CERN’s Large Hadron Collider (LHC). But he says the question of whether the same excess might be explained just as easily by astrophysical phenomena, such as pulsars or a cosmic ray outburst, is even trickier. Hooper emphasizes that, in fact, he thinks the Fermi signal is still “well fit” by models of dark matter being annihilated in the galactic center.

McGaugh disagrees.

“Until both known and un-anticipated astrophysical sources are excluded as reasons for the observed signal, claims about it being due to dark matter are exaggerated at best,” said McGaugh.

As for Boyarsky and colleagues?

Boyarsky notes that his team has acquired more time on the XMM telescope in 2015. And if that doesn’t work, Boyarsky says that likely by mid-2016, Japan’s planned Astro-H x-ray telescope should be able to reacquire his team’s observed x-ray emissions and determine if they are actually due to dark matter.

Dark matter theory persists in part because in cosmic large scale structure, its unseen presence seems to shape the makeup of clusters and superclusters of galaxies along filaments of the cosmic web. Thus, again, without invoking dark matter or alternative theories of gravity, such structure is hard to explain.

“This sanguine attitude has been around a long time,” said McGaugh. “Every five years for the past twenty years, I have heard the confident declaration ‘in five years, we’ll know what dark matter is.’ Obviously, that’s never happened.”

Should we stop looking?

Milgrom says detection efforts should continue in earnest; to simply make it clear that dark matter is not there.

When will there be a clear tipping point away from dark matter theory?

“For some, it will never happen,” said Milgrom, who made that shift years ago, when he proposed Modified Newtonian Dynamics (MOND) – an alternative theory of gravity which alleviates the need for dark matter.

Years ago there was essentially just one “robust” dark matter candidate, namely WIMPs (Weakly-Interacting Massive Particles), says Milgrom. And because WIMPs failed to show up at the Large Hadron Collider and in direct-detection experiments, he says, there is now a whole host of very different dark matter candidates on the table.

As Milgrom hints, whole generations of cosmologists have invested such time, energy, and vast sums of money into identifying this elusive matter that dreams of its existence are likely to die hard.