How did the universe begin?

During the 1970s and 1980s, various observations (notably of galactic rotation curves) showed that there was not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is not normal or baryonic matter but rather dark matter. In addition, assuming that the universe was mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe is far less lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now a widely accepted part of standard cosmology due to observations of the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and x-ray measurements from galaxy clusters. In August 2006, dark matter was definitively observed through measurements of colliding galaxies in the Bullet Cluster. This and other detections of dark matter are only sensitive to its gravitational signature; no dark matter particles have yet been observed in laboratories. However, there are many particle physics candidates for dark matter, and several projects to detect them directly are underway.