Dark matter as dark energy

Velocity-dependent interactions in a fundamental string-dominated universe lead quite naturally, with reasonable assumptions on initial conditions, to an accelerating expanding universe without assuming the existence of a cosmological constant. This result also holds generically for a universe dominated by moving extremal black holes, owing to a repulsive velocity-dependent force. This interaction, however, does not preclude structure formation. Here we discuss a toy model including both ordinary and extremal matter, in which the latter accounts for dark matter while simultaneously acting as effective dark energy. Eternal acceleration is once more seen to arise in this case.