What Is Dark Matter? The Hunt For Universe’s Most Elusive Substance Continues

hubble deep field — Hubble eXtreme Deep Field "XDF" (2012) view — except for a few stars, every speck of light is an entire galaxy — some as old as 13.2 billion years. NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team

Dark matter — the mysterious and ever so elusive stuff that makes up 85 percent of the total mass of the universe we live in. We have been looking for it ever since we came to know of its existence, but so far, the hunt has been frustratingly fruitless.

Currently, the hypothetical Weakly Interacting Massive Particles, which are believed to interact with normal matter through gravity and the weak nuclear force, are the leading candidates to explain the composition of dark matter, but what class of particles these WIMPs belong to is not yet known.

Scientists think gamma rays can help reveal the presence of some of the types of proposed dark matter particles. However, so far, searches carried out using NASA’s powerful Fermi gamma-ray space telescope have failed to reveal any convincing signals.

“We've looked for the usual suspects in the usual places and found no solid signals, so we've started searching in some creative new ways,” Julie McEnery, Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement outlining the findings of three recent studies.

"With these results, Fermi has excluded more candidates, has shown that dark matter can contribute to only a small part of the gamma-ray background beyond our galaxy, the Milky Way, and has produced strong limits for dark matter particles in the second-largest galaxy orbiting it,” she added.

The first study looked for gamma ray signals from the Small Magellanic Cloud — the small satellite galaxy orbiting the Milky Way at a distance of roughly 200,000 light years — while the second one used over six year of Fermi data to analyze the extragalactic gamma-ray background — the diffuse and pervasive radiation that some models predict may arise from distant interactions of dark matter particles.

Both drew a blank, insofar as a conclusive detection of dark matter particles is concerned.

smc_dm_split — The Small Magellanic Cloud (SMC), at center, is the second-largest satellite galaxy orbiting our own. This image superimposes a photograph of the SMC with one half of a model of its dark matter (right of center). Lighter colors indicate greater density and show a strong concentration toward the galaxy's center. Ninety-five percent of the dark matter is contained within a circle tracing the outer edge of the model shown. In six years of data, Fermi finds no indication of gamma rays from the SMC's dark matter. Dark matter, R. Caputo et al. 2016/ background, Axel Mellinger, Central Michigan University

“There is very little room left for signals from exotic sources in the extragalactic gamma-ray background, which in turn means that any contribution from these sources must be quite small,” Marco Ajello from the Clemson University in South Carolina — the co-author of the second study — said in the statement. “This information may help us place limits on how often WIMP particles collide or decay.”

Another class of possible dark matter candidate particles are axions — particles that scientists believe are capable of converting into gamma rays and back again when they interact with strong magnetic fields. A third study, also published in Physical Review Letters, looked for these tell-tale distortions in gamma-ray signals coming from the NGC 1275 — the central galaxy of the Perseus galaxy cluster.

“While we don't yet know what dark matter is, our results show we can probe axion-like models and provide the strongest constraints to date for certain masses,” Manuel Meyer from Stockholm University, who led the study, said in the statement. “Remarkably, we reached a sensitivity we thought would only be possible in a dedicated laboratory experiment, which is quite a testament to Fermi.”

While the hunt for dark matter has so far come up empty-handed, these studies, based on data gathered by the Fermi telescope, have helped scientists narrow down the search parameters for candidate particles.