  Motivated by string-theoretic swampland conjectures, the existence of a dark
fifth dimension, whose size is roughly 1 -- 10 microns, has been proposed. A
great deal of supporting evidence has been presented, and definitive
experimental tests are likely to be carried out. The basic idea is that the
four-dimensional spacetime that we observe lives on a brane that is localized
in the dark dimension. This short note points out that there are two distinct
ways to realize such a scenario in string theory/M-theory. In the one
considered previously the dark dimension is topologically a circle and our
observable 4d spacetime is confined to a brane that is localized in a GUT-scale
region of the circle. An alternative possibility is that the dark dimension is
a line interval with branes attached at each end. This option would imply the
existence of a parallel 4d spacetime microns away from us!
