The final blow to the Aristotelian universe came in the 1970s, when astronomers led by Vera Rubin of the Carnegie Institution conducted research on the rotation of galaxies. The research led to the startling but seemingly inescapable conclusion that as much as 90 percent of the universe must be composed of matter that has not yet been detected. This matter has come to be called "missing mass" (see below), or "dark matter," but conceivably it may be not so much dark as transparent, neither shining nor casting a shadow, and signaling its presence only by its gravitational pull on other matter/energy. Scientists continue to develop theories to predict the nature of dark matter, and experimentalists are placing detectors wherever they can in their attempts to catch this elusive quarry.
There is no more compelling question in cosmology. Not only has Earth long since been displaced from its central position, but the stuff of which it is madeÑas are humans and all other life-formsÑappears to be different from the stuff that makes up most of the universe.
Perhaps the least-recognized development of modern cosmology is its transformation from a field dominated by theory to one dominated by experiment. Most central questions are now within experimental reach, at least indirectly. Less than two centuries ago the French philosopher Auguste Comte stated categorically that humanity could never know the material composition of stars. Today cosmologists routinely conduct experiments that take them 15 billion light-years away in space and time, to the very edge of the visible universe. The major questions now being explored by cosmologists are described below.