The Story of the Lambda
Back in 1915 everyone thought that the universe (which was thought to only consist of our galaxy) was static. This was based on 1) the fact that there were no detectable changes in the positions of the stars and 2) some residual Aristotelian traditions about the heavens being perfect and unchanging.
When the Einstein equation was applied to the universe, the unacceptable prediction was either an expanding or contracting universe. Despite the density of the universe being unknown, Relativity said that a static universe was not possible regardless of how you adjusted the parameters. Einstein had an easy fix for the problem: add a little something to the equation. The lambda term that he added (which he called the Cosmological Constant) was able to balance things such that a static universe was possible.
This state of affairs lasted until 1929 when Edwin Hubble found that the universe was in fact expanding. Einstein later called his addition of the Cosmological Constant his "biggest blunder." Take the lambda out and the equation and you get a nice, expanding universe.
In 1998 the plot took another twist. Until then, cosmologists figured that the Big Bang had (somehow) thrown the universe apart and gravity would be pulling inward and slowing the expansion. The only question was whether the initial throw was hard enough that things would keep expanding forever (like an object launched from earth hard enough that it escapes earth's gravity) or whether the expansion would eventually end and the universe would begin contracting.
The 1998 discovery was that the expansion was accelerating. This was completely unexpected and remains in large part unexplained. Some sort of Dark Energy is the leading candidate, and interestingly enough the discovery has led to the resurrection of the Cosmological Constant. Not only can it be used to stabilize the universe, but it can also account for (mathematically) an extra push to accelerate an expanding universe. Quite the powerful little letter.
Comments
But the static universe was something very deep seated in the minds of cosmologists at the time. After Hubble's discovery of the expansion of the universe a new theory known as the steady state universe was advanced in order to preserve many of the ideas of the static universe, but now with areas where matter was continuously being created in order to account for the expansion. Cosmologists have long had a religious zeal toward their current view of the universe.
Even if it wasn't kosher to introduce an undetectable physical entity to stabilize the universe, apparently it was acceptable to introduce an ad hoc term into our fundamental equations.