What type of astronomical observation shattered the decades-old expectation that the universe's expansion was slowing down due to gravity?

Answer

Observations of distant Type Ia supernovae

In the 1990s, observations of Type Ia supernovae, which act as standard candles, revealed that the expansion of the universe is accelerating rather than slowing down.

What type of astronomical observation shattered the decades-old expectation that the universe's expansion was slowing down due to gravity?
universematterdark matterdark energycomponent