[cambalache]

You have to differentiate 2 things.

The expansion of the universe, which is known since Hubble observations.Although interesting, it can be explained assuming the big bang (a big assumption of course, but with plenty of experimental evidence).

The second thing,, and the truly mind-blowing one, it is that not only the universe is expanding, but that the expansion is accelerating!! The usual date for this discovery is 1998. This is like throwing a baseball upwards and instead of it keeping its speed or de-accelarating (because of gravity and friction) the ball instead starts to move faster and faster. You would suspect the existence of an unknown energy that is powering this acceleration. That is what is called dark energy , which for all intents and purposes it is a giant black box with a question mark right now.