We are slow little creatures. It has just taken a woman all of a minute to stroll fifty yards along the street outside - not very quick, you must admit. In the same time, a little gull, a visitor from the north, has winged back and forth catching a crop-full of bread from me, and the racket of a workman's laborious hammering at a metal plate at the top of the street has traveled about twelve miles, and the image of all four of us would have had time to travel twenty times to the moon and back, or an eighth of the distance from here to the sun.
To put this in perspective of size in the general scheme of things, the light from the centre of our little island galaxy, and all that it has in store for us to remedy the aberrations and perversions of the genetic sports that evolving life engenders now and again, has been travelling towards us since before last year, and before the slow days of your infancy, and before the beginnings of the last century, from before Shakespeare or Jesus or the Greeks or Egyptians, from before the dawning of civilisation and the invention and infliction of money, from before metal, back a full twenty-three thousand years to the very depths of the last ice-age - without stopping, and - of course - at the speed of light. And we are only half-way out from the centre. - Got that? Well, keep it in mind for the time being.
Have you a tape measure? It isn't long enough, but imagine that it is.
Now put just the very tip of your thumbnail on the first gradation. A millimetre thick, more or less?
Right, now on this scale that thickness is the total diameter of our galaxy, which light takes approximately one hundred thousand years to cross. That's right, since all our ancestors were way back in the early Stone Age contesting survival and appetite with lions and sabre-teeth and woolly rhinos and such, and without prospect of any easier way of life for eighty or ninety-odd thousand years or two thousand-odd of their goodish lifetimes of trial and error and struggle.
The nearest comparable galaxy - Andromeda, or M31 - is at about 25 millimetres or one inch along this tape, and happens to be approaching us at about three hundred miles per second. I will mention that it is due to arrive in about two thousand million years and will make some changes here, if you really crave something to worry about.
Look along the tape. At a distance on this scale of approximately one hundred and fifty metres space will be extending, carrying the galaxies within it there, at about the same speed that space conducts light, so that any light the galaxies emit will be red-shifted to be invisible - But here we are referring to the matter of mere size, so more of that another time.
But those galaxies were in that position fifteen thousand million years ago, and were accelerating, so their present position has to be four hundred and fifty or even nine hundred metres away along this tape - or more, in which our hundred thousand light-year-diameter galaxy, remember, is the thickness of our thumbnail. And considering that this nine hundred metres would be just the radius, the probable diameter would likely be twice that, even if that notional 'big-bang'* happened to be true - Otherwise more, much more.