Just think of them as product of phase transitions, think for example when ice pass to water, the fine threads of ice that remains in water. Then, some cosmological theories propose that the early universe suffered some phase transition, product of
what topological deffects were formed.
Habitually divided in 4 categories:
-cosmic strings
-cosmic walls
-magnetic monopoles
-textures
And yes, there's still research. This paper
http://arxiv.org/abs/hep-ph/0406174
Status of particle physics solutions to the UHECR puzzle
Authors: M. Kachelriess
Comments: 15 pages, 8 eps figures, contribution to the special issue "Ultimate energy particles in the Universe" of Comptes Rendues (eds. M. Boratav, G. Sigl)
Report-no: MPP-2004-1
Journal-ref: C.R. Physique 5 (2004) 441
"The status of solutions to the ultra-high energy cosmic ray puzzle that involve particle physics beyond the standard model is reviewed. Signatures and experimental constraints are discussed for most proposals like the Z burst model and topological defects (both allowed only as subdominant contribution), supermassive dark matter (no positive evidence from its key signatures galactic anisotropy and photon dominance), strongly interacting neutrinos or new primaries (no viable models known), and violation of Lorentz invariance (possible)"
examine the possibility that UHECRs are emitted from topological deffects, and talks about some new classes of topological deffects, the Monopolium M,
and cosmic necklaces, for example