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The picture on the left shows the appearance of the whole of the Earth's surface as if it were experiencing a single night. It traces the night-time output of light by major cities of the world and it is easy to identify the brightest lights. From the astronomial point of view the picture is also very instructive. For it shows how light is not a faithful tracer of where people are located. Light traces 'money' rather than people. Great areas of Africa, India, and China display relatively little luminosity compared with less populated regions of Europe and North America. The astronomical universe has an analogous property. Until about 25 years ago astronomers created their perspective on the large-scale structure of the universe from pictures of the distribution of luminous galaxies. But now we know that the bulk of the mass in the universe resides in a non-luminous form whose precise identity is still unknown.

This 'dark matter' reveals its presence through the gravitational pull it exerts on the luminous matter and the motions of the luminous objects are consistent with the presence of a large amount of dark material. Astronomers face the problem of trying to determine whether the distribution of light in the universe is a reliable tracer of where the mass resides. Most astronomers expect that lots of light means lots of mass because light is emitted by stars which tend to form in places where the density of mass is much higher than average. But the beautiful image of the light of the world cautions us about assuming too readily that light faithfully traces mass.

The image on the right shows the spiral galaxy NGC 4414 which has ten times as much dark matter in its outer regions as that which is indicated by the light shown in the photographic image.

Text by John Barrow