In science, theories bring together a large body of observations, natural laws, hypotheses, and inferences into coherent, well-supported, and testable explanations that help us make sense of the world in which we live. From a scientist's perspective, facts and theories are not mere components of a hierarchy of certainty. Facts are observations, and theories are structures of ideas that explain observations. In a very real way, then, a theory holds considerably more weight than just a fact does.
In the study of life forms, facts include fossils, their locations, and their measurements. Theories may be proposed to explain these facts. In describing his life's work, Charles Darwin cited two distinct accomplishments: establishing the fact of evolution, and proposing a theory to explain its mechanism, natural selection. That living things have evolved over time is a fact, supported by a wide body of evidence—the fossil record. But there are various theories concerning the mechanism of evolution, evolution by natural selection being one such theory that has stood the test of time.
But rather than being conclusive and absolute, scientific theories are tentative. With new discoveries and experimental techniques, scientific theories are refined. Because scientific theories are not absolute, they invite critical analysis and direct new research. It is through testing and re-testing that a theory's central supposition may be strengthened, or even discredited.
Take this example from history. Before the 16th century, the traditional view of the universe was adopted from the ideas of the ancient Greek astronomer, geographer, and mathematician Ptolemy. That is, for 1400 years, people generally believed that Earth was the center of the universe and that all other celestial bodies revolved around it. However, before his death in 1543, Copernicus proposed a rival idea: that of a Sun-centered universe. Working in the early 1600s, Galileo sought to inquire if Copernicus had it right. Using a relatively new technology, a telescope, Galileo was able to gather quantitative evidence in support of the Copernican theory. He found many facts that contradicted the geocentric view, including that Jupiter had satellites orbiting it, and that Venus was orbiting the Sun. Through his guided research and careful observations, Galileo recorded evidence that would eventually overthrow long-held beliefs about the universe.