A scientific theory is a well-substantiated explanation of some aspect of the natural world that is based on empirical observations, experimentation, and testing. In other words, a theory is an idea or model that has been developed to explain a set of observations and can be tested through scientific methods. A good scientific theory must be testable, falsifiable, and capable of making predictions.

One of the most famous examples of a scientific theory is the Theory of Evolution by Natural Selection proposed by Charles Darwin in the mid-19th century. This theory explains how species change over time as a result of natural selection. According to this theory, organisms that are better adapted to their environment are more likely to survive and reproduce than those that are not well adapted.

The Theory of Evolution by Natural Selection has been supported by numerous lines of evidence from various fields including genetics, paleontology, biogeography, and comparative anatomy among others. The fact that this theory has stood the test of time for over 150 years indicates it is a well-substantiated explanation for the diversity of life on Earth.

Another example of a scientific theory is the Big Bang Theory which explains how the universe began around 13.8 billion years ago with an explosion-like event. This theory was proposed in the early 20th century after Edwin Hubble discovered that galaxies were moving away from each other at increasing speeds.

The Big Bang Theory has been supported by various lines of evidence such as cosmic microwave background radiation, which is thought to be leftover radiation from the Big Bang itself; observations of distant supernovae; and measurements of galaxy clusters among others.

In conclusion, scientific theories are essential tools for understanding and explaining complex phenomena in nature. They are based on empirical evidence and must be testable, falsifiable, and capable of making predictions. Theories like the Theory of Evolution by Natural Selection and Big Bang Theory have stood the test time as they continue to inform scientific research and discoveries.