big-bang theory: (cosmology) the theory that the universe originated sometime between 10 billion and 20 billion years ago from the cataclysmic explosion of a small volume of matter at extremely high density and temperature
theory: a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena
cladistics: a system of biological taxonomy based on the quantitative analysis of comparative data and used to reconstruct cladograms summarizing the (assumed) phylogenetic relations and evolutionary history of groups of organisms