What is Dark Matter?

The galaxies in our Universe are not exclusively made up of the stuff we can see, but are held together by the gravitational pull of so-called dark matter. We shouldn't assume that everything is equally conspicuous and shouldn't therefore be surprised by this discovery.

The image of the Earth at night from space at first glance reveals no discernable pattern, but a closer look discloses many identifiable features — light from major metropolises, oil wells alight in the Middle East, and the warm glow from millions of wood-burning stoves in India. These beacons in turn outline the familiar pattern of the coastlines and continents. But most things on Earth do not shine, and were this the only available snapshot of the Earth, we would have a rather skewed and incomplete picture of the terrain. The majestic mountains of the Andes, Rockies, and Himalayas for instance would be hidden from view.

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