Dark energy is responsible for the expansion of space, but it does not affect the space within galaxies due to the dominance of gravitational forces. While dark energy exerts a force throughout the universe, its influence is negligible in dense regions like galaxies, which remain stable and do not expand significantly. The expansion of space is minimal within these gravitationally bound systems, as the forces holding them together counteract the expansive effect of dark energy. The expansion of the universe is a result of the initial conditions set during its formation, which are still not fully understood. Overall, while space expands, the internal dynamics of galaxies prevent noticeable expansion among their constituent matter.