Here we review dark energy, the component that causes accelerated expansion of the universe. We start by reviewing the history of this fascinating discovery, describing in detail how type Ia supernovae were used to measure the expansion rate and find that the expansion is speeding up. We then outline modern evidence for the existence of dark energy, how dark energy is parametrically described, and what its phenomenological properties are. We review the cosmological-constant problem that encapsulates the tiny size of dark energy relative to expectations from particle physics. Next we introduce physical candidates for dark energy, including scalar fields and modified gravity. We end by explaining the controversial anthropic principle, and describe the possible future expansion histories of the universe dominated by dark energy.

This chapter explains functions and modules. Functions perform some operation. They can operate directly on data or indirectly via other functions. All the niceties of different kinds of functions are explained. In addition, the topic treats the concept of modules, bits of code that are available to you or other programmers. The chapter shows how you can use existing modules in your own code and how you can write your own modules for others. Finally, the chapter goes into the proper way to document your functions and modules.