Ever since Hubble's discovery of the redshift of distant galaxies we know the universe is expanding. However, gravity still acts, and since all of the galaxies pull on themselves (gravity is attractive), we expect the expansion to rate to decrease. In fact, the deceleration parameter, q, was defined, and soon enough, measured. It is a positive number, showing the expansion rate is decreasing (because it is defined as a deceleration parameter, not acceleration parameter.
In one of the most surprising observation since Hubble's, it now appears that the expansion rate is increasing. The amazing story is this: Up until about 5 billion years ago, the deceleration parameter was positive, just as we expect. But about 5 billion years ago, about the time Earth was formed, something spectacular happened. The deceleration parameter became NEGATIVE, meaning the rate of expansion of the universe is INCREASING.