heat death of the universe

Heat death is the idea that one day all of the energy in the universe will slowly run out, which will cause everything to slowly cool down until all the stars, planets, and galaxies are eventually gone. The universe is constantly expanding and creating new stars, planets, and galaxies, but at a certain point all of the energy will run out and the universe will end. This process will take hundreds of billions of years, but eventually all energy will be gone and the universe will be dark and silent.