Ether is a gas that the Eliksni need to survive, produced by the Ether cores inside Servitors. It also helps them grow, and the size of an Eliksni directly correlates with how much Ether they consume. The inside of an Eliksni’s body is extremely pressurized (at least at the head), so the head explodes if it’s pierced. Humans and Awoken cannot breathe Ether, and would suffocate if trapped in a room filled with Ether. Ether does not cause brain damage (this was revealed by Failsafe), so there’s no harm if a human or Awoken breathes it in. But lack of oxygen would cause brain damage and kill a person, so only breathing Ether is a bad idea. Because the Eliksni need Ether to survive, we can assume that it contains something that their bodies need (like nutrients or certain elements). Other than that, Ether really is just a gas. If it contains something like nutrients for the Eliksni, that would explain why it causes growth in Eliksni bodies. We don’t know how Servitors produce Ether, or how the Eliksni survived before they invented Servitors. Servitors are simply highly-advanced AI created by the Eliksni. Because of the melee attack that large Servitors use (where they drain/absorb you), we can guess that Servitors produce Ether by absorbing some form of energy or matter (possibly only organic matter, but the large amount of time some Eliksni have spent isolated in space makes this unlikely). We know that the Eliksni farmed crops. It’s possible that Ether (or the stuff the Eliksni needed to survive) was originally produced by some plants/crops on the Eliksni home world (like how Earth plants produce oxygen), and that these crops were farmed by the Eliksni. https://www.ishtar-collective.net/cards/mystery-the-vault-of-glass?highlight=Pink Edit: [spoiler]It’s most likely that Ether is simply the air of the Eliksni home world. We know that the Eliksni ruled over many worlds (or “many stars”), so it’s likely that Servitors were invented to produce Ether so that the Eliksni could survive when offworld. This theory also makes the most sense, because it’s the only way we can explain the Eliksni existing. If Ether wasn’t already naturally occurring in the air of their planet, it would have been impossible for a species to evolve that depends on breathing Ether. If this theory is true, then the crops the Eliksni grew were for something else (for example, the crops could have produced something used to make clothes, like cotton). This theory potentially contradicts other lore which seems to describe Ether as being something other than the air of their home world. This theory could also imply that the Eliksni only needed to breathe the air of their home world to survive, which is pretty insane (it’s possible, like with plants, but also very bizarre to imagine in an animal).[/spoiler]

According to mainstream science it doesn’t exist.

Its basically eco from Jack and daxter.