Dark energy

Dark energy is the name given to the universal 'fifth force' identified by astronomers during Earth's late 20th century. Like electromagnetism or gravity, dark energy influences all matter in the galaxy, and its effects can be seen on both cosmological and subatomic scales. The repulsive effects of dark energy are the primary reason the expansion of the universe is gradually accelerating.

Dark energy is manipulated using the extremely rare material element zero. When subjected to powerful electrical currents, element zero releases dark energy that can be harnessed to create mass effect fields, effectively raising or lowering the relative mass of all objects within the field. With the eezo nodules and natural electrical impulses in their nervous systems, biotics can generate and wield dark energy biologically, but the effort is physically demanding.

Dark energy and mass effect fields have practical uses across a wide range of manufacturing and technology fields. Most significant is the enabling of FTL travel, which has opened up the vast distances of space to exploration and colonisation.

Source

 * Galactic Codex: Essentials Edition 2183