Section 4.3.2: Dark Energy
Dark energy is thought to constitute over 70% of the mass-energy density of the Universe and be uniform throughout space and time. Current theories associate it with the vacuum in space. The uniformity in time means that it does not change with passing time as the Universe expands. The uniformity in space means that there are no local effects and it has a wider effect across the Universe as a whole. This leads to a repulsive force, causing an acceleration of the expansion of the Universe. The speed of expansion can be measured through observations of red shift. These measurements are used to estimate the quantity of dark energy.
ACTIVITY: Watch this video to learn more about dark energy