For us to see something it has to scatter light from a source (eg bulb, sun) into our eyes.
But space (or the empty bits between starts and planets) doesn’t contain anything so light just passes straight through. If you look at a bit of space therefore, it is dark (apart from the points of light that are stars – which send light rays in our direction).
At first it seems like an easy question – dark is just the absence of light, so if there is nothing in a given direction to emit light (e.g., a star) or to scatter it (e.g., a planet, or piece of dust) then space will seem dark. But if the universe is infinite, shouldn’t we see stars in all directions.
Maybe the universe is too big for light to reach us from every direction, maybe the light from distant stars is just too dim to detect, or maybe there is enough dust in space to absorb light from very distant sources. So maybe the answer isn’t so obvious.
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