If you want, see how this would work as really beginner-level introduction. I'm hearing misconceptions about expansion---not necessarily from the present company, more generally in lay public. People ask what is expanding? how can galaxies recede faster than light? if space is boundaryless then it must be infinite and if it is infinite how can it expand? So if it expands it must have a boundary and there must be "space outside of space", right? You may have heard fallacies like that. Can you define the key cosmo concepts in a nutshell and do it CONCRETELY enough so beginner has something definite he/she can picture? Some permanent mental images to hang on to? For concreteness I've included numbers in this thumbnail sketch. So many numbers might be counterproductive, not sure about that. Instead of trying to say what space "IS", I concentrate on the Cosmic Microwave Background. Redbelly once observed that from a certain POV it is the CMB which is expanding. In an operational practical sense, I think. Anyway here it is, feedback welcome. There are seven points: ====== 1. We live in a remarkably uniform soup of ancient light. A cubic meter has about 413 million ancient photons from around year 370,000. A typical wavelength is nowadays about one millimeter. Microwave ripples about as fine now as wrinkles in your skin or the teeth of a comb. When emitted in or around year 370,000, they were much shorter wavelengths: light similar to sunlight but just a bit more orange colored. 2. If you move in some direction at some speed relative to the soup, you will detect a brighter microwave background spot in that direction. The temperature will be fractionally warmer by the fraction which equals your speed divided by the speed of light. That is the doppler effect. Solar system is moving at 1/8 of a percent of speed of light in a certain direction. We see the ancient light just 1/8 of one percent warmer than average in that direction. For an observer at cosmic rest, not moving relative to this soup of photons, the brightness or temperature of the light is isotropic (meaning: same in all directions). It is the same in all directions to within one part in 100,000---uniform to within one thousandth of one percent. 3. Models of the universe run on cosmic time, as could be clocked by observers at cosmic rest anywhere in space (but not so close to a concentration of mass like a black hole that it would appreciably slow their clocks down.) Given similar clocks and thermometers, stationary observers all over the universe measuring the same temperature of ancient light would estimate the same age of universe--IOW share a common time. 4. Probably the most widely used idea of distance at cosmic scales is proper distance defined at some specified instant of cosmic time. This is simply the distance as it would be conventionally measured if you could freeze things at a particular moment. Of special interest are distances between pairs of stationary or nearly stationary objects (whose individual motions relative to the background of ancient light are small enough to be neglected). 5. The ancient light was emitted (in year 370,000) from matter which was then 42 Mly from what became our galaxy, and which is now 45 Gly from us. Both the distance to that matter and the wavelengths of the light have been enlarged by the same factor (slightly over 1000-fold) while the light has been in transit. The temperature of the light has fallen off by the same factor, as its wavelengths have stretched out. 6. What is meant by expansion is that, at any given moment, distances between stationary observers (i.e.pairs of observers at cosmic rest) and wavelengths of light in transit are growing at the same universe-wide percentage rate. This is estimated to be currently 1/144 % per million years and to be headed for a longterm limiting rate of 1/173% per million years. The two basic things that the standard LCDM cosmic model is about are: A. the dynamical behavior of this percentage growth rate: how large it has been in past, how rapidly it has declined, what longterm rate (we think 1/173%) it is tending towards, and B. the curvature of space. We know it is nearly zero, and exactly zero would mean that the radius of curvature (somewhat like a car's turning radius) would be infinite. However the curvature measurements so far are consistent with either infinite radius of curvature, or just very long (on the order of 100 Gly or larger). 7. So far no convincing evidence either that space has any boundary or that the distribution of matter is not approximately uniform throughout. So for simplicity we don't "make up stuff" about some higher dimensional "space outside of space" or empty space beyond some imagined "limit" of matter. The cosmic model, based as it is on GR, would become needlessly complicated if one tried to include such features---and why include them absent solid evidence? Space not having boundary does not imply anything about its volume---the overall volume of a boundaryless space can be either finite or infinite.