No strings have been observed directly (my personal opinion is that cosmic strings are more hypothesis than theory at this point). Strings are thought to manifest themselves in classical space as fundamental particles (bosons and fermions), and the vibration frequency of a string determines the type of particle (a lower vibration frequency means less energy in the string and therefore a lower mass for the particle).

Dark matter does not create negative gravity; in fact, the most obvious signs of its presence are its gravitational effects (non-Newtonian orbital velocities of stars in galaxies, gravitational lenses, etc.). The force responsible for the accelerating expansion of the universe (negative gravity) is dark energy.
Source(s):
http://www.pbs.org/wgbh/nova/elegant/resonance.html
http://www.superstringtheory.com/basics/basic4.html
http://en.wikipedia.org/wiki/List_of_particles

http://www.astro.cornell.edu/academics/courses/astro201/dm_evidence.htm
http://www.sciencephotogallery.com/pictures_1041243/dark-matter-gravitation...
http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html

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