The gravitational interaction of antimatter with matter or antimatter has not been conclusively observed by physicists. While the consensus among physicists is that gravity will attract both matter and antimatter at the same rate that matter attracts matter, there is a strong desire to confirm this experimentally- although simple algebra shows that the presence of two photons with positive energies following electron/positron annihilations observed frequently in nature is extremely strong evidence that antimatter has positive mass and thus would act like regular matter under gravity.