Dark Matter and the Owens Valley Radio Observatory
Neutral hydrogen gas in a spiral galaxy generally extends far beyond the visible stars, and radio observations that can detect this hydrogen were used to study galactic rotation to great distances. Around 1970, radio astronomers showed that the rotation curves were flatter than Keplerian beyond the optically visible galaxy, and hence that they had to contain a large amount of invisible, or dark, matter. The interferometer at the Owens Valley Radio Observatory was able to study this in greater detail than was possible with the filled-aperture telescopes that existed then, and made the first curves that were unambiguously flat. Within a half-dozen years, however, the field became dominated by more powerful instruments. With these the rotation curves were extended and many more galaxies were measured, and by the late 1970s astronomers generally had become convinced of the need for dark matter.