What is dark energy and why is it tearing the universe apart?

The answer

Dark energy is the name given to the unknown force causing the universe's expansion to accelerate. Discovered in 1998 through observations of distant supernovae, it constitutes about 68% of the total energy in the universe. Unlike gravity, which pulls matter together, dark energy acts as a repulsive force on cosmic scales. If it continues to dominate, the universe will expand forever, with galaxies drifting apart until they're beyond each other's observable horizons. In the most extreme scenario, the 'Big Rip,' dark energy could eventually tear apart galaxies, stars, atoms, and spacetime itself.