But they still don't know what it is.
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The more that scientists stare at it, the more a strange signal from the center of the Milky Way galaxy appears to be the result of dark matter annihilation. If confirmed, it would be the first direct evidence for dark matter ever seen.
Dark matter is a mysterious, invisible substance making up roughly 85 percent of all matter in the universe. It floats throughout our galaxy, but is more concentrated at its center. There, a dark matter particle can meet another dark matter particle flying through space. If they crash into one another, they will annihilate each other (dark matter is its own antiparticle) and give off gamma rays.
To search for a dark matter signal, astronomers use NASAs Fermi Gamma-Ray Telescope to map the gamma radiation throughout the galaxy. Then, they try to account for all known sources of light within this map. They plot the location of gas and dust that could be emitting radiation and subtract that signal from their gamma-ray map. Then they determine where all the stars are and subtract out that light, and so on for every object that might be emitting radiation. Once all those sources are gone, there remains a tiny excess of gamma radiation in the data that no known process can account for.
The more we scrutinize it, the more it looks like dark matter, said astrophysicist Dan Hooper of Fermi National Accelerator Laboratory, co-author of a paper that appeared Feb. 26 on arXiv, a website that hosts scientific papers that have yet to go through peer-review.
Since 2009, Hooper has been claiming that this bright signal is evidence of dark matter. According to his teams latest data, the gamma radiation could be produced by dark matter particles with a mass of 30 to 40 gigaelectronvolts (GeV) crashing into one another. A proton is roughly 1 GeV for comparison.
But the galactic center is a tricky place. There are many other gamma ray sources that could be mimicking a dark matter signal as well as yet undiscovered phenomena that might account for the radiation. For the most part, few other researchers have been convinced of Hoopers data. One oft-used counterargument is that the excess gamma ray signal could come from millisecond pulsars dead star cores that spin extremely fast and beam out a huge amount of energy. Astronomers dont yet have a good understanding of how these objects work.
If you need to explain something weird in the galactic center, you wave your hands and say, Millisecond pulsars, said astronomer Doug Finkbeiner of Harvard, another co-author of the new work.
Finkbeiner has long been a skeptic that the excess Fermi telescope signal represents dark matter annihilation. He knows that the galactic center is a strange place full of unexpected phenomena, having discovered in 2010 two gigantic structures spanning 50,000 light-years emanating from the Milky Way, which had gone unnoticed until then. But a more careful look at Hoopers data has started to convince Finkbeiner that there might be something there.
When a galaxy forms, gravitational attraction brings together a huge mass that begins spinning. As they spin, large galaxies cool down and flatten out like a pizza, forming the familiar spiral shape seen in many telescope images. Dark matter, which actually makes up the bulk of a galaxys mass, cant flatten out because it doesnt interact with the electromagnetic force, which would allow it to radiate away thermal energy. It stays in a spherical halo circling the galaxy. So any dark matter signal should come not just from within the galactic plane, but also from above and below it, where stars are few and far between but dark matter is abundant.
The problem is that the galactic center is extremely bright. Its billions of stars give off an incredible amount of light that shines far above and below the plane of the Milky Way. Showing that the gamma ray signal comes from dark matter and nothing else requires extremely precise mapping. But the Fermi telescopes data also happens to be a little blurry at the energy ranges where the dark matter signal shows up. Working with physicist Tracy Slatyer of MIT, Finkbeiner combed through the Fermi data and found a way to throw out the blurriest parts. This left a very sharp map showing exactly that the excess gamma ray signal was coming from areas where few stars should exist.
The answers just got a lot better, said Finkbeiner. It looked more like dark matter and less like pulsars.
The newly sharpened data is making other researchers take notice. We may in the future say this was when dark matter was discovered, said theoretical physicist Neal Weiner of New York University.
A particle mass of 30 to 40 GeV would make this form of dark matter quite interesting, he added, because it is something that could have shown up at the Large Hadron Collider. The fact that it didnt might suggest that dark matter is more complicated than our simplest models predict.
But others remain more skeptical. If the question is, Have we really discovered dark matter? I would really caution to set the burden of proof as high as we can, said physicist Stefano Profumo of the University of California, Santa Cruz.
There are many things that our Earth-bound perspective might not be taking into account, such as differences in the density or energies of cosmic rays in the galactic center, he added.
Even though he is a co-author of the new work, Finkbeiner also remains cautious. Of all the options he is aware of, he thinks dark matter annihilation remains the best explanation for the Fermi telescopes excess signal. But the universe is vast and there likely remain many unknown objects for astronomers to find.
To definitively answer this question, scientists will likely have to study dwarf galaxies, which are up to 99 percent dark matter and contain few other odd phenomena that could mimic a dark matter signal. The Fermi telescope will have to stare at these objects for a few more years before it has enough data to confirm or deny the latest results.
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