#open-question

What is causing the expansion of the universe to accelerate — is it a constant vacuum energy density (the cosmological constant), a dynamical field (quintessence) that changes over time, or a sign that general relativity itself needs modification on cosmic scales?

Why does the observable universe consist almost entirely of matter with essentially no primordial antimatter, when the Big Bang should have produced matter and antimatter in equal amounts that would have mutually annihilated?

Is there a general theory that predicts the statistical behavior of turbulent fluid flow directly from the Navier-Stokes equations, and can fully chaotic dynamical systems be understood from first principles?

What is the invisible 'dark matter' that makes up most of the gravitating mass in galaxies and the universe, and can it be directly detected in a laboratory?

Why do physical processes in our universe overwhelmingly run in one time direction (entropy increasing), even though the fundamental microscopic laws of physics are very nearly symmetric under time reversal?

What does quantum mechanics actually tell us about physical reality — is the wavefunction real, does measurement cause genuine collapse, and could hidden variables restore classical determinism?

When a spinning coin or disk settles flat on a table, why does the frequency of its wobbling/rattling sound rise dramatically just before it comes to rest, instead of dying away smoothly?