When in 1998, while looking to see how fast the expansion of the Observbable Universe was slowing down in the manner of all Outward Expansions, Modern Scientists dug up Einstein's Cosmological Constant (Anti-Gravity) which Einstein himself had denouced in the strongest language possible, calling it 'The greatest blunder in his career.'
Einstein lived another 25 years after this confession, never saying anything about his Cosmological Constant except how much he regretted it.
When you derive the field equations of General Relativity, you naturally get the cosmological constant or lambda as it is more commonly known. However the theory doesn't tell you what value the constant should have.
Einstein then the constant to a certain value in order to make the universe static (neither expanding or contracting), since he found that more aesthetically pleasing. Observational evidence then showed that the universe was expanding, so lambda must be very small. Einstein then realised he was wrong about what value lambda had and admitted he made a mistake.
So we knew lambda was quite small, near zero. In most cases the difference between a small value of lambda and a zero value is negligible. It makes no difference for stellar orbits or black holes or neutron stars. So for the next few decades we mostly worked with lambda set to zero, since the equations are much easier to solve in this case.
Then in 1998, we found lambda wasn't zero. No big deal really, nobody had being claiming or desperately hoping it was zero.
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