The universe is expanding and accelerating — nobody knows why

In 1998, two independent teams studying distant supernovae discovered that the universe is not just expanding — it is expanding faster and faster. The culprit was named 'dark energy', a placeholder for whatever force counteracts gravity on cosmological scales. Dark energy is estimated to make up roughly 68% of the total energy of the universe, yet its nature remains completely unknown. It is the largest unsolved problem in modern physics.

Source: Riess et al., The Astronomical Journal (1998); Perlmutter et al., The Astrophysical Journal (1999)