What mysterious repulsive pressure, discovered through 1998 supernova observations, is causing the current acceleration of cosmic expansion?
Answer
Dark Energy.
Observations in 1998 revealed that expansion was accelerating, which implied a pervasive, repulsive force overcoming gravity on large scales. This unknown driver has been termed Dark Energy.

#Videos
The Expansion of the Universe Made Simple - YouTube
Related Questions
What is the core physical description of the expansion of the universe?What critical observation did astronomer Vesto Slipher first notice regarding light from remote galaxies?According to Hubble's Law, what is the relationship between a galaxy's distance and its apparent recession velocity?In the raisin bread analogy used to describe cosmic expansion, what do the raisins represent?Why do structures like solar systems and galaxies not expand along with the universe?What is the ultimate implication of the universe expanding into a space that is constantly being created?What mysterious repulsive pressure, discovered through 1998 supernova observations, is causing the current acceleration of cosmic expansion?What percentage of the universe's total mass-energy content is currently attributed to Dark Energy?What is the major theoretical hurdle associated with the Cosmological Constant ($\Lambda$) hypothesis for Dark Energy?What is the 'Hubble Tension' currently facing cosmologists?