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The idea comes down to how matter moves. The fact that dark matter doesn't really do anything but suck, gravitationally speaking, means that it doesn't really follow the cool kids around, either. When ordinary matter gets close to another bit of ordinary matter, it says hello. It does the equivalent of standing in the middle of the supermarket aisle having a long conversation about the health and happiness of both parties' electrons. Along with gravity, this meeting doesn't just cause matter to clump together, it also causes it to move together. So in our spiral galaxy, it isn't just the stars that rotate around a common center of mass. All the ordinary matter rotates around that common center of mass, too.

Dark matter isn't like that. It doesn't just ignore ordinary matter; it doesn't even like its own family. Dark matter is that guy who goes to the club to make sure that everyone knows that he doesn't dance. The only reason that dark matter is in the galaxy at all is that it's moving so slowly that it can't escape the gravitational pull of the rest of the matter around it. Indeed, without these slow-moving herds of dark matter sucking everything in, galaxies wouldn't have formed at all.

But even though it's there, dark matter watches the galactic dance from the sidelines. While the stars and gas clouds all move gracefully round each other on the dance floor, dark matter just hangs out, looking sulky.

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Michelson-Morley Experiment and Aether

The MMX experiment was performed at what is now Case Western Reserve University in Cleveland, Ohio in 1887 by Albert A.Michelson and Edward Morley. The experiment used an interferometer to compare the speed of light in perpendicular directions. The point of the experiment was to determine relative motion of matter through the aether wind or luminiferous aether. It was believed light required a medium in order to move, similar to the way sound waves require a medium (e.g., water or air) to propagate. Since it was known light could travel in a vacuum, it was believed the vacuum must be filled with a substance called aether.

Since the Earth would revolve around the Sun through the aether, there would be a relative motion between the Earth and the aether (the aether wind). Thus, the speed of light would be affected by whether the light was moving in the direct of the Earth's orbit or perpendicular to it. The negative results were published in the same year and followed up with experiments of increased sensitivity. The MMX experiment led to the development of the theory of special relativity, which does not rely on any aether for the propagation of electromagnetic radiation. The Michelson-Morley experiment is considered to be the most famous "failed experiment".