The year was 1998 and two highly competitive groups of astronomers were each rushing toward the same goal: they hoped to hunt down the effects of gravitational braking in the universe. Ever since astronomers had accepted the idea of the Big Bang, they had been out hunting for its subsequent cosmic deceleration.
While the Big Bang blows space apart (it literally stretches all points of space-time away from each other), the gravitational pull of matter should, over time, slow down that initial burst of cosmic expansion.
As data was gathered and analyzed, both the Harvard and Berkeley groups were stunned to find no evidence for deceleration. Instead, everything pointed in the opposite direction.
According to observations, the expansion of the universe was speeding up — it was accelerating. Cosmic acceleration became big news.
Which means there exists a Dark Energy pushing the universe outwards:
In 1999, the newly discovered cosmic acceleration made it clear that some form of anti-gravitational energy had to exist. As nothing was known about this energy…it was called Dark Energy
The discovery of cosmic acceleration and Dark Energy upended cosmology almost overnight.
keep reading at Cosmos and Culture
(thx Joseph Armstrong)