Oddbean

Twenty three hundred years ago Aristotle described the universe as a series of concentric spheres made of a crystalline material called "Quintessence" (The Fifth Element). The Sun, Moon, planets, and stars were all embedded into their own individual spheres which rotated around the Earth at different rates. Four hundred years later, the Egyptian astronomer Ptolemy could not reconcile his observations with the circular motions implied by Aristotle. But circles were perfect, and the heavens must also be perfect, so he invented "epicycles". He said that the heavenly bodies each moved in small circles around a point, and that point moved in a circle around the Earth. Over the centuries, as observations improved, Ptolemy's epicycles weren't enough. So more were added. Wheels within wheels. And even they were not enough so other contrivances of speed and angle were added. In the middle of the sixteenth century Copernicus, who believed that God lived in the Sun, moved the Sun to the center and found he could eliminate some of the epicycles and other contrivances used to match observations. But many still remained. In the early part of the following century Johannes Kepler showed that the shapes of the orbits of the planets were not circles, but were ellipses, with the Sun at one focus of the ellipse. Just a few decades later Isaac Newton collapsed all the previous centuries of observation and reasoning down to one simple formula. F=Mm/r^2. A century later, Einstein, stimulated by the work of Maxwell, Michaelson, and Morley showed us that Newton's formula was an approximation of a rather more complicated rule having to do with time, space, and the speed of light. He called it his general theory of relativity. Two decades later, using the best theories of gravity at his disposal, and Hubble's distance laws for galaxies, Fritz Zwicky noticed that the galaxies in the Coma cluster were moving too quickly for gravity to hold them together. He proposed that there must be more material in the cluster than was visible. He called that material, Dark Matter -- a term first coined by Henri Poincaré to describe a different phenomenon. By the 1960s there were many observations that galaxies rotated too quickly to hold themselves together. The speed of the stars at the outskirts of the galaxy move well beyond the escape velocity of the visible matter. Today we conclude that the universe is full of something we call Dark Matter. It forms the seeds of galaxy clusters, and it's shape outlines the webs of those clusters that are woven through space. We think that this dark matter outweighs visible matter by a factor of nearly six to one. And we have not the slightest clue what this stuff might be.