  We have suggested earlier that D-particles, which are stringy space-time
defects predicted in brane-inspired models of the Universe, might constitute a
component of dark matter, and that they might contribute to the masses of
singlet fermions that could provide another component. Interactions of the
quantum-fluctuating D-particles with matter induce vector forces that are
mediated by a massless effective U(1) gauge field, the "D-photon", which is
distinct from the ordinary photon and has different properties from dark
photons. We discuss the form of interactions of D-matter with conventional
matter induced by D-photon exchange and calculate their strength, which depends
on the density of D-particles. Observations of the hydrogen 21~cm line at
redshifts >= 15 can constrain these interactions and the density of D-matter in
the early Universe.
