X-rays of a specific wavelength emanating from the hearts of nearby galaxies and galaxy clusters could be signs of particles of dark matter decaying in space, two independent teams of astronomers report. If that interpretation is correct, then dark matter could consist of strange particles called sterile neutrinos that weigh about 1/100 as much as an electron. However, some other researches are skeptical.
For decades, astronomers and astrophysicists have thought that some sort of mysterious dark matter must provide the gravity that keeps individual galaxies from falling apart. In fact, the current standard model of cosmology indicates that a typical galaxy forms within a vast clump, or halo, of dark matter whose gravity keeps the stars from flying out into space. However, scientists do not know what dark matter is, as they have never detected it by any means other than sensing its gravity.
Now, two teams report possible signs of dark matter particles revealing themselves in another way—by very, very slowly decaying into normal photons. Both groups relied on data from one of the most successful space observatories, the European Space Agency's X-ray Multi-Mirror Mission (XMM-Newton), which launched in December 1999 and is still taking data. Esra Bulbul, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues discovered x-rays of a specific energy—3.5 kiloelectron volts (keV)—shining from 73 galaxy clusters, including the Perseus cluster. The Harvard group also used data from NASA's orbiting Chandra X-ray Observatory launched by NASA in July 1999, as it reports in a paper submitted to The Astrophysical Journal.