The astronomical observations have shown that expansion of the Universe is accelerating. It has been proposed that this acceleration could be due to the presence of repulsive gravity of dark energy such as cosmological constant. In the present standard cosmological model the energy density of the Universe is composed of 73% dark energy, 23% dark matter, and 4% baryons. Based on this model, the dominant component of energy density of the Universe is unknown and mysterious. In this lecture I will give an introduction to the dark energy and cosmological constant. I will also briefly discuss other logical possibilities for dark energy, namely modified gravity and modified matter.
