|The atmosphere that protects us from gamma rays prevents us from directly observing them from the ground.|
A gamma ray is a photon more energetic than an X-ray (more than about 50 keV). Gamma rays are created from nuclear reactions or particle accelerations. Gamma rays are the most energetic photons of the electromagnetic spectrum.
The GLAST Burst Monitor (GBM) is the instrument on Fermi that is specifically designed to detect gamma-ray bursts.
Gravity is the attractive force of an object with mass on another object. The gravitational force between two objects depends on their masses and the distance between them.