Dark matter is a substance about which we know very little. We know more about what it isn't: it can't be dead stars, rogue planets, or wandering black holes, for example. For various reasons, every kind of normal matter has been eliminated from the list, leaving some form of exotic matter that isn't well understood.
But that doesn't mean we know nothing: we actually can map its location on the sky! As light from distant galaxies passes through dark matter, the gravity of the invisible material bends that light, distorting it - this is called a gravitational lens. The bigger the warp, the more dark matter must be there. The Canada-France-Hawaii Telescope Lensing Survey observed over 10 million galaxies, looking for that subtle distortion, and made dark matter maps of four regions on the sky. The result is the image above. For comparison, it includes the full Moon for scale, as well as the largest dark matter map previously made.
Large scale maps of dark matter like this are critical for understanding its distribution, and for figuring out what the heck this stuff is. As it happens, detectors on board the Fermi spacecraft as well as underground in the Large Hadron Collider are on the hunt for the weird particle constituents of dark matter. Very soon, we may know quite a bit more about it.
Credit: Van Waerbeke, Heymans, and CFHTLens collaboration