Some properties of the "String gas" with the equation of state $p = -ρ/3$

We show that the string gas - a perfect fluid with the equation of state $p = -\frac13 ρ$ 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.