It looks like a bunch of slightly blurry, multicoloured fireflies, but while the image above doesn't look impressive at first glance, it certainly improves on more intimate acquaintance. It's actually a whole bunch of black holes at the centres of galaxies, imaged by NASA's Chandra X-ray Observatory.
To be more precise, it's over 1,000 black holes, in a patch of sky around two-thirds the size of the full moon in the southern constellation of Fornax. A patch of sky the size of the full moon at this concentration would contain 5,000 black holes; the entire sky would contain a billion.