Vacuum energy refers to the underlying energy present in empty space, even when no particles are present. This concept arises from quantum mechanics, where particles can spontaneously appear and disappear, creating fluctuations in energy. These fluctuations mean that even a perfect vacuum is not truly empty but filled with virtual particles that briefly pop in and out of existence. This energy has significant implications in cosmology and physics, particularly in the context of dark energy and the expansion of the universe. It suggests that the vacuum of space can exert a force, influencing the behavior of galaxies and the overall structure of the cosmos.