Always day and always night,
Day and night are properties of planets with a sun, but space contains all of these planets, including both the parts in day and the parts in night.
You'll know me when you see the light.
We [mostly] see light through empty space.
But don't look up, don't mess around -
Ceilings and the light sky by day obscure our view of space.
In these words is where I may be found.
Spaces between these words.
Through eternity ever I flowed,
Space has [probably] existed as long as time has.
Beyond the marble you call your abode.
Space surrounds the Earth (the marble) that's our home.
I am warm and I am cold;
Space has no temperature, but places in space will be very hot and very cold.
Visit me if you are bold.
Brave the final frontier.
If you are confused sometimes,
Just relax, read between the lines.
Spaces between the lines.