Final answer:
High redshift arcs around galaxy clusters are examples of gravitational lensing, a manifestation of the bending of light due to mass, allowing astronomers to study the dark matter content of clusters.
Step-by-step explanation:
The high redshift arcs observed around clusters of galaxies at lower redshifts are now thought to be examples of gravitational lensing. This phenomenon is a manifestation of Einstein's General Theory of Relativity, which predicts that massive objects, including both visible galaxies and dark matter, can bend and magnify the light from more distant galaxies behind them.
This bending of light creates streaks and arcs that we observe. Of particular note, gravitational lensing allows astronomers to calculate detailed maps of matter distribution within clusters and has revealed that clusters contain a significant amount of dark matter, often more than ten times the luminous matter.