Many lay persons who want to talk about their ideas for "new physics" beyond general relativity to explain observations made by astronomers use the term "aether" to describe their ideas, as someone leaving a comment at one of my recent posts did.
I am making this post as a public service to discourage aspiring non-professional scientists from doing so (professional scientists already know better). Most people use this term do so out of ignorance of its modern connotations.
The central notion of the idea of "aether" is that space-time is not just "nothing", and that the vacuum itself has physical properties that make it in some sense a real substance. As discussed below, this core concept is not dead.
But, the term "aether" jumped the shark a couple of decades before the Happy Days episode in 1977 that gave rise to this expression aired. Despite the fact that Albert Einstein used the term himself in published works as late as 1930 and that Paul Dirac published a paper using the term to describe a similar concept as late as 1951, this term is no longer in current usage as a scientific term used by legitimate physicists and hasn't been since the late 1950s.
Today, this term is very strongly associated with a very particular kind of 19th century luminiferous aether theory that was definitively disproven with a many experiments conducted by multiple investigators that were replicated with increasing precision from 1810 to 1935. As a result, the luminiferous aether theory is synonymous in contemporary physics writing by professional physicists with pseudoscience. "Aether" is to physicists what Young Earth Creationism is in the fields of biology and geology. Thus, from a P.R./marketing/credibility perspective, it is hard to imagine a worse choice of name for the medium of space-time than aether by anyone trying to seriously and sincerely advance a scientific hypothesis about physics.
While many "new physics" modern gravitation/dark energy/dark matter theories proposed by professional physicists (and indeed general relativity itself) treat the fabric of space-time as something that has properties rather than being "nothing", using the term "aether" for that medium is the rhetorical equivalent of calling yourself a crackpot. This is the perception that people who naively use this term are unintentionally broadcasting to people who read their writings about physics.
Sometimes it makes sense to embrace a word that has become a slur (e.g. "gay" and "lesbian"), but this is not one of those times.
Among professional physicists anywhere in the world in the twenty-teens of the current era, the word "Aether" has connotations that make it basically a dirty word in reputable discussions of physics. Using the word "aether" sincerely, to describe a possible real physical phenomena in a published physics paper or blog post or physics forum comment, is a roughly equivalent writing an official government report or think tank paper or newspaper article describing 2010 census data that uses the word "Nigger" describe African-Americans, the word "Savages" to describe Native Americans, the phrase "Wetback" to describe Hispanics, and the word "Gook" to describe Asian Americans. It destroys your credibility before you have a chance to make a point about the legitimate physics idea that you want to express. Use of the term "aether" brands you as a backward anti-scientific idiot even if this isn't really who you are at all. If you want to make a positive impression you should not make this rookie mistake.
When professional physicists wish to express the notion that space-time itself may give rise to gravitational or inertia effects or otherwise fundamentally re-think of the foundational mechanism of effects observed by astronomers and physicists, they have a great many alternative ways of saying what they mean. A non-professional physicist (or a professional physicist) who wants to express similar ideas should do likewise. Physicists who talk about a theory in which the physical properties of a vacuum or space-time or empty space that are not nothing, often speak of the properties of space-time, manifolds, branes, the physical properties of the vacuum, the inherent curvature of space-time, the background, the cosmological constant, vacuum energy, dark energy, quintessence, scalar fields such as the Higgs field and the inflaton, superfluid vacuum theory, the Dirac sea, Le Sage gravity, a Bose-Einstein condensate, and Mach's principal. But, legitimate physicists almost never use the term "aether" in any sense other than a derogatory or dismissive one.
Given the term aether's illustrious history of use back to the days of Aristotle that it shares with terms like "atom" that have endured the test of time better, this may seem to a would be lay physicist to be a crying shame. But, life is not fair and this is the sociological reality of modern academia and physics. Any amateur physicist who uses the term should be forewarned about this linguistic connotation reality, and should expect the treatment that they will receive when they use it from professional physicists and more sophisticated amateur physicists alike.