Microwave ripples
Figure 2. An image of the cosmic-microwave-background anisotropy measured by the Cosmic Background Explorer satellite over four years. (The dipole anisotropy has been subtracted.) The fluctuations in the background are extremely faint, only 1 part in 100 000, and are the imprint of a density contrast in the early universe. The density ripples are believed to have given rise to the structures of galaxy clusters that populate the universe today. (Picture credit: NASA/COBE)