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.