Dynamis (or Akasha, in the field of Alchemy) is a volatile form of energy that reacts with strong emotions. Dynamis makes up roughly 68% of all known energy in the universe, almost double Aether. However, Dynamis is a far weaker form of energy, and is almost completely nullified by Aether in equal quantities. When Dynamis appears in large amounts in a focused area, its effects are observable. For instance, largely negative emotions of hopelessness or despair will twist and warp the aether of whomever is channeling it, with effects ranging from the affliction Totentanz observed in ancient Hannish myth, which is said to amplify the rage and violence in ones heart, spreading it to those around them, to transforming a person into what is now known as a Blasphemy, completely consuming their aether and turning them into a violent, unthinking beast.
Dynamis can also have an effect on positive emotions. The ancient Hannish Dancers used to wield a power known as Kriegstanz, which would endow those around them with hope, warding off the effects of Totentanz. Adventurers calling upon a fervent desire to protect their allies, defeat their foes or revive their fallen comrades may also experience a Power welling up inside them allowing them to do just that.
In the distant past before the Sundering, the Ancients were made of far denser aether than the people of Etheirys today, and were completely unaffected by Dynamis. This also meant that studying the energy source was far more challenging than studying and channeling aether. There were those, such as Hermes who dedicated themselves to studying Dynamis, which gave way to the creation of Entelechies, beings who were powered by Dynamis instead of aether.
Upon completing their mission to search the cosmos for other forms of life, the Entelechies known as the Meteia became overwhelmed by sorrow and despair at the amount of death and suffering beyond their star. When this was channeled into Meteion, her Dynamis was completely affected by these powerful emotions, twisting her into a being that determined that all life must be extinguished to avoid such suffering. She escaped to the far reaches of the universe, beyond anything containing aether where Dynamis was far more abundant, and channeled it towards the planet. While this didn't effect the Ancients directly, they lost control of their creation magicks, causing fear and panic to overwhelm the people, which in turn caused the appearance of more Blasphemies. This effect was only halted when the Convocation of Fourteen came together and called upon Zodiark, who shielded the star with a powerful layer of aether. Much further into the future, after the Sundering and Zodiark was destroyed, removing the barrier of aether, people were able to halt the effects of Dynamis if they could master their emotions.
Many of the fictional properties of Dynamis, such as it's weakness versus Aether and being "68% of all known energy in the universe" are deliberately similar to the real-life concept of dark energy. Several other related cosmological concepts are obliquely referenced in Endwalker, including Superposition, Quantum entanglement, and the heat death of the universe.