Air is an invisible, unobtainable, untouchable, but extremely common block. It is everywhere where there isn't a solid block. Some may think that the game doesn't render anything in these open spaces, but proof that air blocks exist come from a certain glitch where, after mining a block, the name Air replaces the name of the material for a split second. This is non-canon to the game of course, but the games use this as a "filler" for spaces without solid blocks.