The COBE experiment, designed to measure the remnants of the initial big bang fireball at the edge of the Universe, also made another very important discovery. Using a device known as a 'differential microwave radiometer' (http://space.gsfc.nasa.gov/astro/cobe/dmr_synopsis.html) it became apparent that matter and energy were not distributed evenly in the very early Universe. Researchers found that the Cosmic Microwave Background was very slightly variable on the scale of one part in 100,000. That is, there are very tiny temperature fluctuations which can be measured by the differential microwave radiometer. This unevenness in the distribution of matter and energy has been described as the 'lumpiness' of the early Universe. The more scientific terminology is that the Universe shows 'anisotropy'. It is worth noting that elsewhere you will read that the Universe is homogeneous or isotropic. The difference in viewpoint is all a matter of scale. On a large scale the Universe is homogeneous and everywhere has a temperature of 2.73°K at its edge. On a fine scale, however, variations in temperature have been measured. It is this fine detail which is also now seen to be important.
The significance of the variation in the intensity of the Cosmic Microwave Background is that it shows how matter and energy were distributed when the Universe was still very young. It is thought that these early inhomogeneities subsequently developed into the regions in the present Universe where there is matter (galaxies and galaxy clusters) and the regions from which matter is absent (space). The early inhomogeneous distribution of matter also reflects an inhomogeneous distribution of density. These initial density differences gave rise to small differences in gravitational forces which began to draw matter together.
Follow the link http://space.gsfc.nasa.gov/astro/cobe/dmr_image.html to view some of the images of the early Universe measured by the COBE satellite. Some of the images are contoured in microkelvins (µK), that is millions of a degree Kelvin, such is the level of sensitivity of the measurements.