Including time, the physical environment has four dimensions, all, as far as our senses can tell, extending out to infinity in each direction.
In the case of space we have become used to mind-bending numbers of light years describing the distance of the more remote bodies, and beyond there we give up, whatever theories mathematicians may produce. We can use a symbol to denote infinity, but we cannot in our minds grasp the idea of endlessness in all directions, nor of curving space; it seems fair enough that light and gravity should be able to curve, but not space.
Much the same difficulty applies to time, although here we have critical dates, which seem in a way less remote than some of the distances – age of the solar system about 4.5 x 10_9 years; date of the Big Bang about 15 x 10 years. Let us take our space/time ship back through the Big Bang, carefully screening its systems from cataclysmic event. What then? Nobody is going to convince us that time started ticking at some specified instant in the Big Bang. Nor that it is some cloud system analogous to the surface of a sphere. We are so infinity-orientated despite our intellectual problems with infinity that we expect to see time stretching away forever backwards and forwards, possibly occupied by a series of Big Bangs and Anti-Bangs if we care to speculate. We can no more conceive of a margin to space and time than we can grasp the idea of their infinite extent. The intellectual problems remain much the same if one Big Bang, or a succession of them or continuous creation, is the theory in vogue amongst those we hope to be the best able to judge.
As organisms, we live at a particular scale of space and time that suits our biological convenience. Our intelligence has given us the ability to be able to think in terms of much larger and much smaller scales of both, though, and to appreciate that there is no special sanctity to our own or any other scale. With no valid reference point, small and large become relative terms, and the infinitely small and infinitely large pose the same sort of problems to our comprehension as the other infinities. As we move to larger scales however, matter seems to become much less substantial. The ‘ultimate’ particle of the atom is made of smaller components and these possibly of smaller ones again, all with a lot of space between them and mostly comprising forces and charges reacting constantly with each other. What would one sense if one was able to enlarge the smallest sub-atomic particle to the size of a football pitch, and then to do the same a thousand-fold again? Would this sub-sub-microscopic world be of any lesser significance than our own?