A former cosmological conundrum, a value judgement resulting from the offended aesthetic sensibilities of some physicists.

A possible interpretation of Albert Einstein's theory of General Relativity, an expanding universe, proved itself true over the succeeding decades with Edwin Hubble's observations, and evolved into the Big Bang theory with Penzias and Wilson's detection of cosmic microwave background radiation. A expanding Universe with a fixed amount of matter seemed to indicate that gravity would slow down the expansion, and possibly stop and reverse it, eventually ending in a Big Crunch. The mathematics of this theory depended upon the current rate of expansion (the Hubble Constant) and whether the overall density of matter in the Universe exceeded some critical density or not.

Scientists, always in search of an elegant solution, wanted to find that the density of matter in the Universe was exactly the critical density. Others preferred to believe in a Big Crunch. Unfortunately, the amount of observable luminous matter in the Universe is estimated to be only about 1% of the required value!

This irked many scientists, and during the 1970s and 1980s they began to search for the "missing matter", better known as dark matter or cold dark matter. Observations of spiral galaxies showed that there was indeed a lot of dark matter out there; many theories were advanced to speculate as to its nature: Some of these theories are indeed still viable, and dark matter is still a meaningful sub-discipline of physics.

However, the idea that matter might be "missing" was mooted by the discovery that the expansion of the Universe is accelerating rather than slowing down. All of the physicists' dreams of a perfectly balanced cosmology were swept away.

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