Number 177 (Story #2), May 3, 1994 by Phillip F. Schewe and Ben Stein THE DENSITY OF EXTRAGALACTIC DEUTERIUM has been measured for the first time by scientists using the new 10-m Keck Telescope in Hawaii. In particular, these astronomers measured the ratio of deuterium to ordinary hydrogen in a distant gas cloud by observing the absorption of light from an even more distant quasar as it passes through the cloud. The D/H ratio, 2.5 x 10**-4, is much higher than the ratio measured in our own galaxy. An accurate measurement of this ratio may help to determine what fraction of the suspected dark matter in the universe consists of baryons, particles (such as the proton) comprised of three quarks bonded together. The Keck observations also provide an upper limit, 13.5 K, on the temperature of the cosmic microwave background as it would have appeared at the location of the cloud, at a redshift of 2.9. This is consistent with a theoretical estimate of 10.7 K. (A. Songalia et al., Nature, 14 April 1994.)