  Motivated in part by string theory, we consider the idea that the standard
LambdaCDM cosmological model might be modified by the effect of a long-range
scalar dark matter interaction. The variant of this widely-discussed notion
considered here is suggested by the Brandenberger-Vafa picture for why we
perceive three spatial dimensions. In this picture there may be at least two
species of dark matter particles, with scalar ``charges'' such that the scalar
interaction attracts particles with like sign and repels unlike signs. The net
charge vanishes. Under this condition the evolution of the mass distribution in
linear perturbation theory is the same as in the LambdaCDM cosmology, and both
models therefore can equally well pass the available cosmological tests. The
physics can be very different on small scales, however: if the scalar
interaction has the strength suggested by simple versions of the string
scenario, nonlinear mass concentrations are unstable against separation into
charged halos with properties unlike the standard model prediction and possibly
of observational interest.
