Fermi's Large Area Telescope has detected 1873 gamma rays out in space. Most come from objects such as pulsars or blazars. But for 600 of those rays, scientists have no idea where they're coming from.
Not that they don't have their suspicions about where these mystery rays are coming from. After all as NASA's science blog points out, space is littered with dark matter we can't see.
"Some of the mystery sources could be clouds of dark matter – something that's never been seen before," speculates Thompson.
"About 85% of the gravitational mass of the universe is dark matter. The stuff we see makes up the rest. Dark matter is something that pulls on things with the force of its gravity but can't be detected in any other way. It doesn't shine – doesn't emit or scatter light – hence the adjective "dark."
"Astronomers cannot detect dark matter directly using optical or radio telescopes. But dark matter just might shine in gamma rays."
Scientists believe that gamma rays, an energy-packed light source so strong we can't detect it visibly, could be generated when two dark matter particles collide, but have no way to confirm this. And that sucks, because I hate suspense. Especially suspense that has no forseeable resolution. [Science@NASA]