Why is space called space? As in, who named it?

This is an interesting question. First, it should be noted that while we aren't able to absolutely say who first used the word "space" to refer to the vacuum of outer space, we are able to say with some confidence that the earliest usage of the word in that context may have occurred in 1667 in the epic poem Paradise Lost by John Milton. This implies that it was at least already being used somewhat commonly with this meaning. 

Prior to Milton, the word "space" has its history in English stretching back as far as the 14th century, variously meaning "place, extent of time, territory, distance between two points." The word descends from the Old French word "espace", ultimately from the Latin "spatium", with more or less the same meaning as its English descendant. Though we now know that outer space is not entirely "empty", we can see the relationship between the concept of space as the empty void between the heavenly bodies and the idea of the word space referring to the empty area between places or points in time.