We lack direct experimental probes of physical forces in the dark sector. Can the anomalies present in the standard LCDM model be remedied by a modification of the physics of the dark sector? We discuss various scenarios in which long range scalar interactions can play a significant role in structure formation. In particular, simulations show that a long range scalar interaction with a Yukawa force law has many desirable features: it causes voids be more nearly empty, suppresses accretion onto galaxies at low redshift, and produces an early generation of dense dark matter halos.