Astronomers question the eternal expansion of space. Dark energy is a mysterious force in the universe.
It causes space to expand faster and faster and galaxies to move away from each other. If the force remains the same in eternity, the matter will become more and more rarefied and the universe will become darker and colder until no further life is possible anywhere. However, if the dark energy changes, the universe may face a different fate. It could either become so strong that it tears apart galaxies, then planetary systems and even atoms. Or all all matter could become one point faster and a kind of upside-down Big Bang.