  The case grows ever stronger that the average density of matter, ordinary and
dark, is less than the critical density required for a flat universe. However,
most of determinations of the mass density have been dynamical, hence sensitive
only to matter which is clustered at or below the scale of the observed
dynamical systems. The density may still be critical if there is a dark matter
component which is relatively smooth on the scales of galaxies or clusters.
Thoughts on this matter have focused on the possibility of an effective
cosmological constant or vacuum energy. In this letter we examine an
alternative possibility - that there is a second component to the dark matter
which has a repulsive self-interaction. We show that given even very weak
self-repulsion, this dark matter would remain unclustered. While this repulsive
alternative is perhaps aptly named, it is arguably at least as palatable as a
cosmological constant.
