wolram said:
This may be nieve but i have not found an explanation yet, i guess the background is the medium needed for light to travel by and for gravity to permeate, but as aether theories have been ruled out this leaves only space it's self as the background AFAIK.
In this case for some theories space must have been the first (thing) to exist?
The word has no one unique meaning. When people talk about background they mean different things and you have to study the context to tell what they mean.
What many people mean by it is an assumed background
metric geometry. A continuum---a set of points representing space or spacetime can be either stiff or floppy. What gives a continuum a rigid shape is having a metric or distance function defined on it. Then you can measure angles and triangles and areas and volumes etc. If there is a fixed metric on a continuum you can do geometry on it.
If a continuum has no metric then it is shapeless and floppy and stretchy. You can't do geometry.
There is a BIG ISSUE in physics which is when you build a model of space or of spacetime should you
1. start with a completely floppy continuum and not assume any fixed metric at all, at the beginning. No preconceptions, just distribute some matter in your continuum and see what kind of metric evolves. (this is how General Relativity does it) Or should you
2. start with a continuum already equipped with a fixed metric----typically representing a flat zero-curvature vanilla geometry---and then add stuff bit by bit and allow slight modifications, like ripples on a basically flat pond surface.
So in the context of that ongoing argument, background is shorthand for background
metric assumed at the start, in constructing a model. 1915 General Relativity manages to not need a background metric, so it is independent of background geometry.
I think you are asking about something else. You say you guess "the background is the medium needed for light to travel by and for gravity to permeate" and that is certainly one possible definition but I never heard anyone use the word background to mean that. However one says it, asking
what it is is a good question. You could use a different word so as to reduce the risk of confusion.
You could say "empty space" or "vacuum" or maybe continuum.
whatever light travels thru and is shaped by the gravitational field.
Calling it background would also seem like a good idea if it were not for the risk of getting mixed up in a loud discussion of something entirely different---where people mean something else by it.
Frank Wilczek has a new book (The Lightness of Being) which is entirely about the modern physics understanding of the vacuum. He never once calls it "background", though. He has various things he calls it. Sometimes he says "the entity we call empty space".
His point is that we call it empty space but we probably shouldn't think of it as empty.
I conclude from reading the book, and everything else I've seen on the subject, that we don't know. We don't know what is the [whatever light travels thru and is shaped by gravity]. We don't know what the background is (if you insist on calling it that).
We don't know the best way to think about empty space. What Wilczek says in the book is that HE sees ways in which the LHC results will help greatly to grasp empty space better. He explains how, for him, knowing how many Higgs-like particles there are and what their properties are will help figure out how empty space works.
He explains how seeing some supersymmetry would be a big help in comprehending the vacuum.
The book points out that there are immediate questions. Some things that are known about empty space---he refers to layers of knowledge. He says we know some layers but there are further layers about which he is itching with curiosity. He makes lists and some items have question marks beside them. It is in some ways it is an impatient book. He is very eager to get some LHC results.
He doesn't talk about the Big Bang, or string theory, his focus is on ordinary empty space and he takes a kind of bottom-up approach to it (based on those experiments done in the past at earlier colliders and those he expects to be done in the future.)