Then in 1929 Edwin Hubble discovered that the universe wasn't static but actually expanding. The repulsive force from the Big Bang over 15 billion years ago countered the force of gravity and Einstein's fudged cosmological constant was discarded forever. Thus the universe lived happily ever after.Recent observations have added an interesting epilogue to the story which shows that some kind of cosmological constant may yet exist after all.It all began in 1998 when a team of astronomers at the Lawrence Berkeley National Laboratory made the inconvenient discovery that the universe actually accelerating while it expands. This isn't supposed to happen. Issac Newton's laws of momentum state that an object can only accelerate when being pushed by an outside force. The catch is that without Einstein's constant, there isn't supposed to be any outside force any more. The Big Bang sent everything in the universe flying apart and the force of gravity was supposed to gradually slow that expansion. Instead, things seem to be speeding up.There has to be some massive unseen force throughout the cosmos making it expand ever faster. Physicists call this exotic force " dark energy " because while it can't be seen directly, its effects can be observed. Scientists know still don't know what dark energy actually is or where it comes from, but they've been measuring its effects on distant galaxies and stars. Recent measurements made using NASA's Chandra X-ray Observatory show that dark energy has been a constant force throughout the history of the cosmos.A constant force in the cosmos? A cosmological constant perhaps?