Chaos energy was a little-understood phenomenon before well into Era-3.  Most of the time it appears to be just part of the random background noise of the universe, generally cancelling its self out.  On rare occasions it can manifest unpredictable results.  It can alter people or objects in bizarre ways, open unstable wormholes, manifest objects or beings, or do any number of other things.  These results; however are so rare that individual incidents are almost always ignored as an isolated fluke of nature.  Bigger incidents can spawn chain-reactions that cause multiple strange events, but were so rare before Era-2, as to basically be unknown outside of obscure legend and wild conjecture.

Chaos Energy is often generated as a by-product of Spatial Compression technology, and places and times with little use of Spatial Compression tend to have only very low levels of Chaos Energy.  When a spatial compression device generates negative energy to create an Alcubierre Field, it pushes some of the energy out of our universe into hyperspace, allowing Chaos Energy to flow back in through the breach in the local brane.

Throughout Era-0, Chaos Energy was uncommon enough that its manifestations were written off as flukes of nature.  Even in the Empire of 1,000,000 Suns, where Spatial Compression was used, the local dependence on Stargates made it exceedingly rare.

During Era-1, while Spatial Compression was widely used in the Planetary League, the amount of usage, combined with the relatively low effectiveness of the Spatial Compression devices of the time made Chaos Energy a rarity.  It did become more pronounced toward the end of Era-1, as the technology improved and interstellar travel became more common.

The Great Reckoning was the titanic war that started Era-2, and the massed fleets of the Holy Empire of Man created vast amounts of Chaos Energy.  This is widely seen as the main factor leading to the rise of the Chaos Worlds during this time period.  The Imperial forces of the time spent much blood and treasure battling chaotic entities, but also benefited from a few talented people within their ranks who could sense and manipulate Chaos Energy. Typically those with this ability ended up suffering grave side-effects, or even being wholly consumed by this fickle force.

During the middle part of Era-3, scientists in the Galactic Republic discovered that Chaos Energy was the byproduct of Spatial Compression technology, and was a more-or-less direct manifestation of the unraveling of the space-time continuum itself.  Chaos energy was directly responsible for the Big Fade, a cataclysm in which everything in the galaxy gradually de-materialized into a field of undifferentiated energy at the end of Era-3.

The structure of most organic brains has a minuscule ability to detect and manipulate Chaos Energy, although some beings possess natural talents that allow them to do supernatural-seeming things using it.  Even with little talent, a highly skilled practitioner can perform amazing feats using it if it is present at sufficient levels.

The renowned scientist-turned pirate, Dr. Immanuel Jefferson Cyclone was affected by chaos energy sometime early in his career, while testing an experimental hyper-drive, leading to his very eventful life.

The mysterious group known as the Elect of Zanath would often turn up at the scene of unusual incidents in their secret battle against the negative repercussions of chaos energy eruptions.

The Hand of Eris effectively worships Chaos Energy, seeking out and recruiting or training Chaos Energy users, and dedicating their efforts toward amplifying the Chaos Energy in the universe.