Dark energy is a hypothetical form of energy responsible for the accelerating expansion of the universe. Unlike matter, it has negative pressure, pushing apart the fabric of space-time.
It cannot be directly observed, but its existence is inferred from observations of cosmic microwave background radiation, supernovae, and large-scale structures in the universe.
Its exact nature is still unknown, and some theories suggest it could be a property of space itself or associated with a new fundamental particle.