Although most celestial objects are known to possess
their own magnetic fields, observational evidence has been
lacking for magnetic fields in intergalactic space on scales
larger than those of clusters of galaxies. Since the activity
of astrophysical objects is likely to be subdued inside
intergalactic voids, such regions may possibly have retained
primordial magnetic fields generated at very early epochs,
whose observation could give us insight into the early Universe.
Here we discuss methods to probe weak intergalactic magnetic fields
by searching for delayed secondary gamma rays from highenergy sources
such as TeV blazars or gammaray bursts, and show that
a lower limit on magnetic fields in a void region of 10^{-20} Gauss
can be derived from current data for the blazar Mrk 501.