Even the greatest theories are lacking behind to determine what the mystery actually is. More is unknown than is known. Back in 20th century, scientists were quite sure that the universe is expanding. In theories, they could explain that the universe had to slow down its expansion by the virtue of the gravity as the time went on. But, to their surprise, with Hubble space telescope observations of very distant supernova, scientists realized that expansion of the universe has not been slowing as it was theorized, but actually is was speeding up its expansion. Some mysterious substance was causing it but there is no explanation to this till now! But we have a solution and scientists prefer to call it as dark energy.
Possible explanations of dark energy:
Scientists now thinkthat acceleration of the expansion of universe is caused by some mysterious repulsive force generated by quantum disturbances in otherwise “empty” space. Surprisingly, this force is growing stronger as the universe is expanding. In consideration of its mysterious effects, scientists named it as dark energy. Roughly 68% of the universe is dark energy and dark matter makes up about 27% and the rest comprises of everything observed till date with our instruments that comprises of less that 5% of expanding universe.
Dark energy has been named as fifth fundamental force called quintessence, after the fifth element of Greek philosophers. Scientists have explained it as a dynamic energy fluid or field that fills all of space. Its effect on the expansion is the opposite of that of matter and normal energy.Even if quintessence can be the answer, we really don’t know how it reacts with and how it exists.
Many theorists came up with the idea of the cosmological constant from the discarded version of Einstein’s theory of gravity, which according to him was a repulsive force that opposes gravity and prevents the universe from collapsing in on itself. But Einstein found this to be inappropriate when observations revealed that the universe is expanding and named it as “biggest blunder”!! Taking in dark energy as cosmological constant, we could have an explanation of how space-time is being stretched apart. But even with this theory, we would be clueless as we don’t know how it interacts with the other celestial objects and why this force even exits! Hence the mystery continues.