The universe has been expanding ever since the Big Bang, with galaxies moving away from each other due to the stretching of space itself. This expansion is not due to galaxies moving through space, but rather the space between galaxies expanding. The rate of expansion is measured by the Hubble constant, and recent observations show that the universe's expansion is accelerating.
Scientists theorize that this acceleration is driven by a mysterious force known as dark energy, which makes up about 68% of the universe’s total energy content. Understanding the expansion of the universe is key to unlocking the secrets of cosmology and the ultimate fate of the cosmos.