Scientific theories are explanations of natural phenomena supported by extensive evidence and repeated experimental scrutiny. In everyday language, "theory" is often used to mean a guess or a hunch, but these speculations are more closely aligned with the term "hypothesis." Instead, scientific theories are widely accepted frameworks for how the universe works because they account for existing observations and make predictions that have been rigorously tested.
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Scientific laws describe outcomes of specific conditions, and scientific theories explain why those outcomes occur. Both are supported by observational and experimental data and can each inform one another, but neither can become the other. Instead, additional data can improve each towards more accurate descriptions and explanations of reality.
Facts are observations of the world, and they can be pooled together for a given phenomenon to develop a hypothesis—a proposed explanation consistent with observations that requires further investigation. Experiments and additional data can rule out many hypotheses or graduate a proposed explanation into a widely accepted and validated theory.
Through the acquisition of scientific knowledge, scientists construct explanations of the world, which are rigorously tested and verified until they narrow down to a leading theory of the natural world. However, because of how science works, these ideas will never become factual, as they must always remain open to be iteratively revision based on new data.
Introduced by philosopher Karl Popper, falsifiability holds that a scientific theory must make predictions that could be proven inaccurate by future observations. Explanations that cannot make such predictions are not scientific theories. It is scientists' jobs to seek out data to poke holes in scientific theories, thereby potentially strengthening them if they hold up to scrutiny.
Humoral theory held that everyone's personality derived from different combinations of four bodily fluids: blood, phlegm, yellow bile, and black bile. The phlogiston theory stated that all combustible materials possessed a fire element—phlogiston—that was released and absorbed by the air during burning. When the air became saturated with phlogiston, the fire would go out.
When Thomas Kuhn published "The Structure of Scientific Revolutions" in 1962, he described the history of science as comprising periods of "normal science" where these accepted frameworks—paradigms—gradually accumulate anomalies until they cannot describe nature. Revolutionary science, such as the development of a new theory, results in a paradigm shift, another term Kuhn helped popularize. (Some readers may experience a paywall.)
After confirming that the accuracy of some laws of physics is limited to certain domains, science became more hesitant to categorize identified mathematical relationships, such as quantum mechanics, as unchanging laws. Instead, these recent, complex frameworks are designated as theories to acknowledge that they are conditional and may be part of a larger, more complete future framework.
This interactive concisely describes many highly complex theories ranging from quantum gravity and black holes to dark matter and energy. Supplementary resources, key questions, and possible solutions accompany each description. As of 2026, no developed theory provides an accepted explanation of all observed phenomena in the universe at all scales.
Gradual changes in organisms have been directly observed through artificial selection—humans purposely selecting organisms to breed to pass on specific traits—and in organisms with short lifespans, such as insects and microbes. Analyses of DNA and anatomical structures across different species, as well as fossil records, also reveal ancient shared ancestry consistent with natural selection.
Multiple independent lines of evidence—galactic redshift, element ratios, cosmic microwave background radiation, and large-scale cosmic structure—all point to a single origin about 13.8 billion years ago. Before Edwin Hubble, many astronomers believed the entire universe was contained within our galaxy.
Dive into the dynamic world of science and technology, where curiosity has brought about extraordinary understandings of the universe and creativity has led to the breakthroughs and innovations that have transformed our world. Explore a wide range of topics, from the natural and physical sciences to cutting-edge technologies and the people who shaped them, each of which is accompanied by carefully curated resources meant to inform, engage, and inspire those eager to uncover the nature of reality.