cosmic microwave background
In 1992, data from the Cosmic background explorer showed that the temperature varies by ten parts per million, or thirty microkelvin, from one direction to another across the sky. The anisotropies in temperature and polarisation are thought to represent variations in the density of matter across the universe. These anisotpies are supposed to have been fixed some 14 billion years ago, remaining constant yet polarised!
Nature/ volume 420, 19/26 December 2002, p747.