Earth with city lights (artist's impression)

The stars and galaxies we see in the night sky represent only one sixth of the total matter in the Universe. The remainder is a mysterious component - dark matter - that neither emits nor reflects light. So, for astronomers, the challenge of mapping the Universe has been similar to mapping a continent from just the lights of the cities.

Credit: ESA/Hubble (M. Kornmesser & L. L. Christensen)