Explain what the two teams of astronomers found in the 1990s. what question did their discovery create?

There were many discoveries in the 1990's, one of them was carried out by two independent research teams: the Supernova Cosmology Project and the High-Z Supernova Search (This discovery made them win the Nobel Prize in Physics).

 Until that moment, the astronomers, including these two teams mentioned before, believed that the expansion of the universe was slowing down, but  in their search of supernovas to measure this expansion over time, they discovered that they were wrong: The expansion of the universe was accelerating. But this created another question:  What force is driving this behavior in the universe?