Another mystery facing cosmologists is the accelerating expansion of the universe. In 1929, astronomer Edwin Hubble showed that the universe was expanding, but for this expansion to be justified, there must be much more mass or energy in the universe than we can see. One possible explanation for this was provided by Peebles in a study published in 1984, which suggested that there might be energy driving the expansion of the universe. This theory became valid in 1998, when observations revealed that the universe was not only expanding, but accelerating — a discovery that won the Nobel Prize in Physics in 2011.
Peebles' idea actually revived the "cosmological constant" that Albert Einstein added to the equations of general relativity to explain what he believed at the time - that the universe was in a state of stability and stability - and then removed it from the equation when it became clear that the universe was in a state of expansion. This "missing" mass is nowadays called "dark energy", and according to calculations, it accounts for about 69 percent of the total mass in the universe.
