I don't think it was space. The translations given in
The Silmarillion for 'Ekkaia' are 'The Outer Sea' and 'The Outer Ocean'. An early
Silmarillion quote says this:
Quote:
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And coming then down to Valmar (Manwë) summoned the Valar to the Ring of Doom, and thither came even Ulmo from the Outer Sea.
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Ulmo being there would certainly seem to indicate it was a water ocean.
Another quote from
The Silmarillion:
Quote:
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But by Anar the waters of the Outer Sea were made hot and glowed with coloured fire.
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After the Fall of Númenor I believe it was an ocean still. Only Valinor and Eressëa were physically removed at that time, and it could still be 'The Encircling Sea' compassing all the lands, much as looking at the Earth from space shows all oceans to really be one.