Antipodes

Apparently, from Greeks to at least the 16th century, there was a real concern over whether there were antipodes, i.e. people on the other side of the globe with their feet turned toward ours. The church worried whether that postulation implied a division in the human race; Augustine concluded it must therefore be nonsense. In "Good Friday, 1613: Riding Westward," John Donne wonders if, from God's point of view, he could "behold that endless height which is / Zenith to us, and our Antipodes, / Humbled below us?"

From the OED:
antipodes, n. pl.

1. Those who dwell directly opposite to each other on the globe, so that the soles of their feet are as it were planted against each other; esp. those who occupy this position in regard to us. Obs.
2. fig. Those who in any way resemble the dwellers on the opposite side of the globe. Obs.
3. Places on the surfaces of the earth directly opposite to each other, or the place which is directly opposite to another; esp. the region directly opposite to our own.
4. a. transf. The exact opposite of a person or thing. (In this sense the sing. antipode is still used.)

The antipodes seem to be our opposites, but not malicious; they're not our evil counterpart, just our significant other. A bit like today's 'dark matter.' (Antipodes sounds about as scientific as 'dark matter' does any way... ) Maybe these antipodes are just our shadows.