Some properties of the "String gas" with the equation of state $p =   -\rho/3$
  We show that the string gas - a perfect fluid with the equation of state $p = -\frac13 \rho$ possesses rather interesing properties. In Friedmann universes its presence can can change the observable topology of the space; in the spherically symmetric spacetimes it produces rather bizzare geometries and in a way its influence on the rotation curves mimics the dark matter effects. 
Alexander Yu. Kamenshchik and Isaak M. Khalatnikov
gr-qc