A substance so common that it makes up three-fourths of all the matter in the universe, but which has never been seen because it is invisible, may have been captured at an old Minnesota iron ore mine. The announcement, which if confirmed would be a hugely important discovery in the field of physics,was made by several American laboratories on Thursday. They said the find was made by detectors below ground at the bottom of the mine. Scientists have theorized the existence of dark matter for more than 70 years. In 1933 the astronomer Fritz Zwicky argued that galaxies could not be held together unless an invisible material provided gravity. His experiments showed that the Coma galaxy cluster must have had a mass of about 400 times the amount that could be visually observed. A 1980 paper based on spectrograph studies hypothesized that more than half of the mass of all of the galaxies in the universe would have to be comprised of dark matter because, otherwise, Newton's Law of Gravity would not be universal. Scientists say they may have captured mysterious "dark matter"