But to this day, no one knows what dark matter actually is. It’s strange even calling all that “normal” matter, because in the grand scheme of the cosmos, normal matter is the rare stuff. Scientists call this unexplained stuff “dark matter,” and they believe there’s five times more of it in the universe than normal matter - the stuff that makes up you and me, stars, planets, black holes, and everything we can see in the night sky or touch here on Earth. Most of the matter in the universe is actually unseeable, untouchable, and, to this day, undiscovered. It turns out all the stars in all the galaxies, in all the universe, barely even begin to account for all the stuff of the universe. You can listen to the episode below (and subscribe wherever you listen to podcasts!). I’ve been thinking about this feeling - the awesome, terrifying feeling of smallness, of the extreme contrast of the big and small - while reporting on one of the greatest mysteries in science for Unexplainable, a brand new Vox podcast about unanswered questions in science. The beautiful challenge of stargazing is keeping this all in mind: Every small thing we see in the night sky is immense, but what’s even more immense is the unseen, the unknown. And while these stars seem astonishingly numerous to our eyes, they represent just the tiniest fraction of all the stars in our galaxy, let alone the universe. They appear as tiny points of light, but they are massive infernos. NASA’s Goddard Space Flight Center, Greenbelt, Md.If you go outside on a dark night, in the darkest places on Earth, you can see as many as 9,000 stars. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States. NASA’s Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. “The more information we have, the more information we can put into models of our own galaxy.” “We still have a lot to learn about the gamma-ray sky,” Caputo said. While more observations are necessary to determine the source of the gamma-ray excess, the discovery provides an exciting starting point to learn more about both galaxies, and perhaps about the still elusive nature of dark matter. “It’s like living in a world where there’s no mirrors but you have a twin, and you can see everything physical about the twin.” “Our galaxy is so similar to Andromeda, it really helps us to be able to study it, because we can learn more about our galaxy and its formation,” said co-author Regina Caputo, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. But telescopes view Andromeda from an outside vantage point impossible to attain in the Milky Way. While Fermi can make more sensitive and detailed observations of the Milky Way’s center, its view is partially obscured by emission from the galaxy’s disk. The similar discovery in both the Milky Way and M31 means scientists can use the galaxies as models for each other when making difficult observations. “M31 lets us see how cosmic rays behave under conditions different from those in our own galaxy.” “We don’t fully understand the roles cosmic rays play in galaxies, or how they travel through them,” said Xian Hou, an astrophysicist at Yunnan Observatories, Chinese Academy of Sciences in Kunming, China, also a lead scientist in this work. Studying Andromeda may help scientists understand the life cycle of cosmic rays and how it is connected to star formation. Because cosmic rays are usually thought to be related to star formation, the absence of gamma rays in the outer parts of M31 suggests either that the galaxy produces cosmic rays differently, or that they can escape the galaxy more rapidly. For example, M31 emits few gamma rays from its large disk, where most stars form, indicating fewer cosmic rays roaming there. Now that Fermi has detected a similar gamma-ray signature in both M31 and the Milky Way, scientists can use this information to solve mysteries within both galaxies. To test whether the gamma rays are coming from these objects, scientists can apply what they know about pulsars from observations in the Milky Way to new X-ray and radio observations of Andromeda. Because M31 is 2.5 million light-years away, it’s difficult to find individual pulsars. Some pulsars emit most of their energy in gamma rays. One teaspoon of neutron star matter would weigh a billion tons on Earth. These spinning neutron stars weigh as much as twice the mass of the sun and are among the densest objects in the universe. Credit: NASA/DOE/Fermi LAT Collaboration and Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSFĪnother possible source for this emission could be a rich concentration of pulsars in M31’s center.
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