In 1998, scientists discovered that the expansion of the universe has been accelerating. What force is causing this phenomena? dark energy Hubble's law the Doppler effect background radiation

dark energy caused this phenomena

The answer is Dark Energy. It is a mysterious force that seems to counteract gravity. While cosmologists had expected that the universe would be slowing in its expansion due the force of gravity, the universe was discovered to be actually accelerating (observed through a technique called Doppler shift effect by the Hubble Telescope). Dark energy is responsible for this acceleration in universe expansion.