Gamma rays are the highest energy photons on the electromagnetic spectrum.  Their wavelength is similar to the size of an atom, and when two of them collide they tend to produce a matter-antimatter particle pair.  They represent energy high enough to synthesize the fundamental particles of matter, and are produced in the highest energy environments in the cosmos.  The interchange of matter and energy works both ways, so one of the ways gamma rays are generated is through annihilation of a matter-antimatter particle pair.  Looking back to the beginning of the universe it gives us the earliest ‘chicken or egg’...