The Universe is full of dark matter. 85 percent of all the matter in the Universe is made of this stuff, and we have absolutely no idea what (or where) it is. It's one of the biggest fundamental unanswered questions in science or cosmology or what have you, so it's a little bit exciting that there's a new candidate for dark matter: a type of "sterile" neutrino that might be what's causing a strange X-ray signal to show up around large galaxy clusters.
Dark matter is just matter (stuff) that has mass (and therefore some gravitation influence on the stuff around it) but doesn't otherwise interact with other stuff. It doesn't absorb or emit radiation, and it doesn't have any charge, which makes it (so far) practically impossible to observe or measure. Neutrinos are subatomic particles that nearly fit the bill for this: they're also nearly impossible to detect, since their lack of charge means that they can pass straight through ordinary matter. In fact, you have 65 billion of them flying through every square centimeter of your body right now. The only way to stop them (or detect them) is if one of them has the bad luck to run smack into the nucleus of an atom, whereupon it smashes itself to bits and we can detect it.
Normal neutrinos are moving too fast, and aren't heavy enough, to account for all the dark matter in the Universe. But "sterile" neutrinos are something else. If they exist, they'd be even harder to detect, but when they decay, they might emit X-rays like the ones that NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton have recently detected coming from galaxy clusters. Since we're not even sure that these neutrinos exist, however, it's going to take a lot more work from astronomers and physicists to figure out if these X-rays have anything to do with neutrinos (or dark matter) at all. We may have to wait for the launch of Japan's fancy new Astro-H space-based X-ray telescope in 2015 before we can find out anything for certain.
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