What counterintuitive conclusion was drawn from HST's precise measurements of distant supernovae in the late 1990s?

The expansion of the universe is not slowing down due to gravity, but is instead actively accelerating.

One of the most profound and surprising outcomes of the HST's refined measurements, particularly using distant supernovae as distance markers, was the discovery concerning the rate of cosmic expansion. Current expectations, based only on the gravitational pull of matter, suggested the expansion should be decelerating over time. However, the high-precision data collected by the HST indicated the opposite: the rate of expansion is increasing, or accelerating. This counterintuitive finding necessitated the proposal of a dominant, unknown repulsive force permeating space to explain this observed acceleration.