Silly question! Everyone knows what space is, we learnt about it in school and take it for granted. Space is a vacuum, the void between the earth, the moon and the stars. We know that space is not actually a complete vacuum, but contains minute amounts of gas and dust. What's more, light and radio waves travel through space without need of a medium (unlike sound, which needs matter for transmission) at a fixed maximum speed (300,000 kilometres per second). Space is the passive backdrop of the universe, everything happens in space, so space can be essentially ignored. On the infinitesimal scale space is the 'gaps' in between atomic and sub-atomic particles.
We can introduce into this very simple understanding of space, a few small complications based on the insights of Albert Einstein. This space that we all take for granted, which is measured in three dimensions, height, width and depth has one other non-spatial (i.e. you can't see it) dimension of time. This space-time combination is not the same everywhere, and is in fact bent or distorted by gravity. To be more exact you could say that space-time is curved, that this curvature is more pronounced near large concentrations of matter or energy and that it is this curvature of space that is felt as the force of gravity.
Edwin Hubble discovered that the galaxies far away in space were in fact moving away from us. The most accepted explanation is that the universe must be expanding, and expanding equally everywhere. The most common analogy given for this is that of a balloon. Semi inflate a balloon, draw some dots on it and inflate it some more. The dots, analogous to the galaxies, move further apart. Of course this is a two dimensional example to help us visualise a three or four dimensional phenomenon. The important point to grasp here is that the galaxies are not speeding away from us through space, space-time itself is expanding, taking the galaxies with it.
This leaves us with a new picture of space. Space is no longer passive, like the stage upon which an opera is performed. This 'stage' is expanding in size equally in all directions (except in the immediate vicinity of the players) and is causing the principal players to rotate around each other.
Quantum physics gives us yet another description of space. Quantum theory describes all forces as the exchange of 'messenger' particles and electromagnetic radiation as both waves and particles called photons. This description relies on the existence of 'virtual' particles which are extremely short lived, popping rapidly in and out of existence, detectable only by their effects on the 'real' particles. Quantum theory therefore depicts space at the sub-sub-atomic level as being like an energetic 'foam' of virtual particles. (Imagine the virtual particles to be like the bubbles that pop in and out of existence when you pour a glass of lemonade.) Careering through this 'foam' are countless photons of light, radio and x-rays etc.
String theory, which is really the latest and greatest in physics, postulates that space-time doesn't consist of just three dimensions of space and one of time but ten dimensions of space and one of time. Where are the extra seven and why can't we see them? The other seven are 'rolled up' into the seven-dimensional equivalent of a sphere, called a seven-sphere or 'hyperball', invisible because of its infinitesimal size. Every point in our observable three-dimensional space is a seven-dimensional 'hyperball'. According to string theory the title for this article should be 'How Far In Is Out Where?'
Nothing isn't simple.
Further reading: SUPERFORCE by Paul Davies Revised 1995 Edition Penguin Books
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