Comments concerning a hypothetical mesoscopic dark dimension

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 space–time that we observe is confined to 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 space–time 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 “end-of-the-world-branes” at each end. The latter option would imply the existence of a parallel 4d space–time, with the same gravitational forces as ours, but otherwise its own physical laws, microns away from us!