Scientists may have "seen" dark matter for the first time thanks to NASA's Fermi Gamma-ray Space Telescope. If so, it would mark the first direct detection of the most mysterious substance in the universe, Space.com reports, writes **UNN**.
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The theory of dark matter was put forward in 1933 by astronomer Fritz Zwicky, who found that the visible galaxies of the Coma Cluster did not have the necessary gravitational influence to prevent the cluster from flying apart. Then, in the 1970s, astronomer Vera Rubin and her colleagues found that the outer edges of spiral galaxies rotated at the same speed as their centers, which would only be possible if the bulk of the mass in these galaxies was not concentrated in their centers, but rather dispersed more widely. Of course, these are not direct observations of dark matter, but inferences made using the interaction of dark matter with gravity, as well as the effect that gravity has on ordinary matter and light. However, thanks to these inferences, astronomers later estimated that all large galaxies are embedded in huge halos of dark matter that extend far beyond the visible matter in galaxies (e.g., galactic halos of stars).
It is estimated that particles of this mysterious substance outnumber the particles that make up everyday matter by a ratio of five to one. This means that everything we see around us every day – stars, planets, satellites, our bodies, the neighbor's cat, etc. – makes up only 15% of the matter in the Universe, with dark matter accounting for the remaining 85%. Adding to the mystery of dark matter is the fact that, because it interacts so weakly or not at all with electromagnetic radiation, it does not emit, absorb, or reflect light. Thus, it is effectively invisible at all wavelengths of light – or, at least, so we thought.
As the publication writes, there is one possibility that would cause dark matter to produce light. If dark matter particles "annihilate" when they meet and interact, similar to how matter and its antimatter counterpart do, then this should produce a shower of particles, including gamma-ray photons, which, although invisible to our eyes, could be "seen" by sensitive space-based gamma-ray telescopes. One of the proposed "self-annihilating" particles believed to constitute dark matter are the so-called "weakly interacting massive particles" or "WIMPS."
A team of researchers led by Tomonori Totani from the Department of Astronomy at the University of Tokyo directed the Fermi spacecraft to regions of the Milky Way where dark matter should accumulate, specifically in the center of our galaxy, and hunted for this characteristic gamma-ray signature.
"Totani believes we have finally found this signature," the publication says.
"We detected gamma rays with photon energies of 20 gigaelectronvolts (or 20 billion electronvolts, an extremely large amount of energy) extending in a halo-like structure toward the center of the Milky Way galaxy," Totani said. "The gamma-ray component precisely matches the shape expected from a dark matter halo."
And this, as the publication notes, is not the only close match. The energy signature of these gamma rays precisely matches those predicted to arise from the annihilation of WIMP particles upon collision, which are predicted to have a mass approximately 500 times greater than protons, the ordinary matter particles found at the heart of atoms. Totani suggests that there are no other astronomical phenomena that would easily explain the gamma rays observed by Fermi.
"If this is correct, to my knowledge, it would mean the first time humanity has 'seen' dark matter. And it turns out that dark matter is a new particle that is not part of the current standard model of particle physics," Totani said. "This indicates an important development in astronomy and physics."
While Totani is confident that what he and his colleagues have discovered is a sign that WIMP dark matter particles are annihilating each other in the heart of the Milky Way, the scientific community as a whole will need more compelling evidence before the book on this nearly century-old mystery is closed, the publication notes.
"This can be achieved when more data is accumulated, and if so, it will provide even more compelling evidence that gamma rays originate from dark matter," Totani added.
The team's research was published on Tuesday (November 25) in the Journal of Cosmology and Astroparticle Physics.