One hundred years ago, Albert Einstein came up with a new theory of gravitation. Later he added to his field equations the cosmological constant which is accountable for the accelerated expansion of the universe. Thus a natural question arises concerning what is the energy source for this expansion. This monograph is about the novel concept of antigravity and antigravitational forces. We present various observational arguments showing the expansion on small cosmological scales. In particular, the Solar system and single galaxies expand at a rate comparable to the Hubble constant. The novelty...